diff --git a/lib/iscsi-command.c b/lib/iscsi-command.c
index 9591b11..f4d12d5 100644
--- a/lib/iscsi-command.c
+++ b/lib/iscsi-command.c
@@ -981,9 +981,9 @@ iscsi_compareandwrite_task(struct iscsi_context *iscsi, int lun, uint64_t lba,
 	struct scsi_task *task;
 	struct iscsi_data d;
 
-	if (datalen % blocksize != 0) {
+	if (datalen % (blocksize * 2) != 0) {
 		iscsi_set_error(iscsi, "Datalen:%d is not a multiple of the "
-				"blocksize:%d.", datalen, blocksize);
+				"blocksize * 2:%d.", datalen, blocksize * 2);
 		return NULL;
 	}
 
diff --git a/lib/scsi-lowlevel.c b/lib/scsi-lowlevel.c
index d766772..331fce5 100644
--- a/lib/scsi-lowlevel.c
+++ b/lib/scsi-lowlevel.c
@@ -1802,7 +1802,7 @@ scsi_cdb_compareandwrite(uint64_t lba, uint32_t xferlen, int blocksize, int wrpr
 
 	scsi_set_uint32(&task->cdb[2], lba >> 32);
 	scsi_set_uint32(&task->cdb[6], lba & 0xffffffff);
-	task->cdb[13] = xferlen/blocksize;
+	task->cdb[13] = xferlen/blocksize/2;
 
 	task->cdb[14] |= (group_number & 0x1f);
 	task->cdb_size = 16;
diff --git a/test-tool/0340_compareandwrite_simple.c b/test-tool/0340_compareandwrite_simple.c
index 535542b..3567205 100644
--- a/test-tool/0340_compareandwrite_simple.c
+++ b/test-tool/0340_compareandwrite_simple.c
@@ -21,6 +21,14 @@
 #include "scsi-lowlevel.h"
 #include "iscsi-test.h"
 
+static void bitwise_invert(unsigned char data[], int len) {
+	int j;
+
+	for (j = 0; j < len; j++) {
+		data[j] = ~data[j];
+	}
+}
+
 int T0340_compareandwrite_simple(const char *initiator, const char *url)
 { 
 	struct iscsi_context *iscsi;
@@ -80,9 +88,11 @@ int T0340_compareandwrite_simple(const char *initiator, const char *url)
 			goto finished;
 		}
 		memcpy(data, task->datain.data, i * block_size);
+		memcpy(data + (i * block_size), task->datain.data, i * block_size);
+		bitwise_invert(data + (i * block_size), i * block_size);
 		scsi_free_scsi_task(task);
 
-		task = iscsi_compareandwrite_sync(iscsi, lun, 0, data, i * block_size, block_size, 0, 0, 0, 0, 0);
+		task = iscsi_compareandwrite_sync(iscsi, lun, 0, data, i * 2 * block_size, block_size, 0, 0, 0, 0, 0);
 		if (task == NULL) {
 		        printf("[FAILED]\n");
 			printf("Failed to send COMPAREANDWRITE command: %s\n", iscsi_get_error(iscsi));
@@ -136,9 +146,11 @@ int T0340_compareandwrite_simple(const char *initiator, const char *url)
 			goto finished;
 		}
 		memcpy(data, task->datain.data, i * block_size);
+		memcpy(data + (i * block_size), task->datain.data, i * block_size);
+		bitwise_invert(data + (i * block_size), i * block_size);
 		scsi_free_scsi_task(task);
 
-		task = iscsi_compareandwrite_sync(iscsi, lun, num_blocks + 1 - i, data, i * block_size, block_size, 0, 0, 0, 0, 0);
+		task = iscsi_compareandwrite_sync(iscsi, lun, num_blocks + 1 - i, data, i * 2 * block_size, block_size, 0, 0, 0, 0, 0);
 		if (task == NULL) {
 		        printf("[FAILED]\n");
 			printf("Failed to send COMPAREANDWRITE command: %s\n", iscsi_get_error(iscsi));
diff --git a/test-tool/0341_compareandwrite_mismatch.c b/test-tool/0341_compareandwrite_mismatch.c
index ca5adc0..645daa8 100644
--- a/test-tool/0341_compareandwrite_mismatch.c
+++ b/test-tool/0341_compareandwrite_mismatch.c
@@ -58,7 +58,7 @@ int T0341_compareandwrite_mismatch(const char *initiator, const char *url)
 	/* write the first 1 - 255 blocks at the start of the LUN */
 	printf("Compare and write first 1-255 blocks (data is not matching) ... ");
 	for (i = 1; i < 256; i++) {
-		task = iscsi_read16_sync(iscsi, lun, 0, i * block_size, block_size, 0, 0, 0, 0, 0);
+		task = iscsi_read16_sync(iscsi, lun, 0, i * 2 * block_size, block_size, 0, 0, 0, 0, 0);
 		if (task == NULL) {
 		        printf("[FAILED]\n");
 			printf("Failed to send READ16 command: %s\n", iscsi_get_error(iscsi));
@@ -85,8 +85,10 @@ int T0341_compareandwrite_mismatch(const char *initiator, const char *url)
 
 		/* flip some bits */
 		data[ (i - 1) * block_size] ^= 0xa5;
+		/* set the write part of the data-out buffer to 1s */
+		memset(data + (i * block_size), 0xff, (i * block_size));
 
-		task = iscsi_compareandwrite_sync(iscsi, lun, 0, data, i * block_size, block_size, 0, 0, 0, 0, 0);
+		task = iscsi_compareandwrite_sync(iscsi, lun, 0, data, i * 2 * block_size, block_size, 0, 0, 0, 0, 0);
 		if (task == NULL) {
 		        printf("[FAILED]\n");
 			printf("Failed to send COMPAREANDWRITE command: %s\n", iscsi_get_error(iscsi));
@@ -153,8 +155,10 @@ int T0341_compareandwrite_mismatch(const char *initiator, const char *url)
 
 		/* flip some bits */
 		data[ (i - 1) * block_size] ^= 0xa5;
+		/* set the write part of the data-out buffer to 1s */
+		memset(data + (i * block_size), 0xff, (i * block_size));
 
-		task = iscsi_compareandwrite_sync(iscsi, lun, num_blocks + 1 - i, data, i * block_size, block_size, 0, 0, 0, 0, 0);
+		task = iscsi_compareandwrite_sync(iscsi, lun, num_blocks + 1 - i, data, i * 2 * block_size, block_size, 0, 0, 0, 0, 0);
 		if (task == NULL) {
 		        printf("[FAILED]\n");
 			printf("Failed to send COMPAREANDWRITE command: %s\n", iscsi_get_error(iscsi));
diff --git a/test-tool/0343_compareandwrite_beyondeol.c b/test-tool/0343_compareandwrite_beyondeol.c
index 9451991..b416a1e 100644
--- a/test-tool/0343_compareandwrite_beyondeol.c
+++ b/test-tool/0343_compareandwrite_beyondeol.c
@@ -57,7 +57,7 @@ int T0343_compareandwrite_beyondeol(const char *initiator, const char *url)
 	/* read 1 - 255 blocks beyond the end of the device */
 	printf("Writing 1-255 blocks with one block beyond end-of-device ... ");
 	for (i = 1; i <= 255; i++) {
-		task = iscsi_compareandwrite_sync(iscsi, lun, num_blocks + 2 - i, data, i * block_size, block_size, 0, 0, 0, 0, 0);
+		task = iscsi_compareandwrite_sync(iscsi, lun, num_blocks + 2 - i, data, i * 2 * block_size, block_size, 0, 0, 0, 0, 0);
 		if (task == NULL) {
 		        printf("[FAILED]\n");
 			printf("Failed to send COMPAREANDWRITE command: %s\n", iscsi_get_error(iscsi));
@@ -97,7 +97,7 @@ int T0343_compareandwrite_beyondeol(const char *initiator, const char *url)
 	/* read 1 - 255 blocks at lba 2^63 */
 	printf("Writing 1-255 blocks at LBA 2^63 ... ");
 	for (i = 1; i < 256; i++) {
-		task = iscsi_compareandwrite_sync(iscsi, lun, 0x8000000000000000, data, i * block_size, block_size, 0, 0, 0, 0, 0);
+		task = iscsi_compareandwrite_sync(iscsi, lun, 0x8000000000000000, data, i * 2 * block_size, block_size, 0, 0, 0, 0, 0);
 		if (task == NULL) {
 		        printf("[FAILED]\n");
 			printf("Failed to send COMPAREANDWRITE command: %s\n", iscsi_get_error(iscsi));
@@ -128,7 +128,7 @@ int T0343_compareandwrite_beyondeol(const char *initiator, const char *url)
 	/* read 1 - 255 blocks at lba -1 */
 	printf("Writing 1-255 blocks at LBA -1 ... ");
 	for (i = 1; i < 256; i++) {
-		task = iscsi_compareandwrite_sync(iscsi, lun, 0xffffffffffffffff, data, i * block_size, block_size, 0, 0, 0, 0, 0);
+		task = iscsi_compareandwrite_sync(iscsi, lun, 0xffffffffffffffff, data, i * 2 * block_size, block_size, 0, 0, 0, 0, 0);
 		if (task == NULL) {
 		        printf("[FAILED]\n");
 			printf("Failed to send COMPAREANDWRITE command: %s\n", iscsi_get_error(iscsi));
@@ -159,7 +159,7 @@ int T0343_compareandwrite_beyondeol(const char *initiator, const char *url)
 	/* read 2-255 blocks, all but one block beyond the eol */
 	printf("Writing 1-255 blocks beyond eol starting at last block ... ");
 	for (i = 2; i < 256; i++) {
-		task = iscsi_compareandwrite_sync(iscsi, lun, num_blocks, data, i * block_size, block_size, 0, 0, 0, 0, 0);
+		task = iscsi_compareandwrite_sync(iscsi, lun, num_blocks, data, i * 2 * block_size, block_size, 0, 0, 0, 0, 0);
 		if (task == NULL) {
 			printf("[FAILED]\n");
 			printf("Failed to send COMPAREANDWRITE command: %s\n", iscsi_get_error(iscsi));