Thank you piotrnik. I now understand why it didn't work, let me explain: Basically, I'm testing with a linux box and a windows seven pc both running btsync 1.3.93. I've generated a random file on the Linux box and let btsync do it thing to obtain the same file on both machines. Adding/removing data anywhere in the file triggers a transfer from the added/removed chunk to the end of the fileReplacing data anywhere in the file (not adding/removing but replacing data bit by bit) triggers transfering only small parts (the replaced data + probably some chunks around)At first, I was adding data at the very beginning of the file, so the whole file was transfered. When adding a few bits at the end of the file or replacing a few bits in the file, only a fraction of the file is transfered (the smallest possible chunk seems to be 32kB). So yeah, btsync definitely does delta transfer, and the chunk size is a multiple of 32kBytes. Just keep in mind that the chunks are calculated from the beginning of the file, so when adding at the beginning, each chunk of the source will be different from the destination file, so a full file transfer will be triggered. Same with adding data somewhere in the file, the first chunks will be the same, but the chunk containing your added data and all the following ones will be different, meaning possibily a big transfer depending where you change your data. Replacing data in the file is ok since only the modified chunks will be transfered. Adding data at the end of the file is also ok since only the new chunks will be transfered.