The mechanism of breakage of polyethylene terephthalate/cotton staple yarns has beeninvestigated by studying the fibers removed from actual yarn breaks. Techniques have been devised that show the strain history of the PET fibers at a break and give the fraction of broken PET and cotton fibers. It is postulated that yarn breakage takes place in two stages; an initial rapture, followed by breakage of PET fibers spanning the gap formed. These techniques were used to examine yarns broken during mill spinning. It appears that the mill breaks occurred by fiber slippage before the yarn was fully developed, probably at the drafting stage. -- AATA