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Poly(ethyleneterephthalate)(PET)is ather moplastic of high-volu me applications, andisiden- tified as Nu mber 1inthe ResinIdentification Code onsingle-use packages. The ester bondsinthe poly mer chains are prone to hydrolysis, but the rate of hydrolysis is extre mely lo w at roo m temperature. Here weshowthathydrolysiscausesPETtogrowcracksevenatroomtemperature and under lo w loads. The hydrolytic cracks greatly outrun erosion. When PET is sub merged in water andsubjectedto a fixedload,the crack velocityincreases with p H. At highloads,the crack gro ws rapidly, and hydrolysis is negligible, so that the crack gro ws with substantial plastic defor mation andthefracturesurfaceisrough. Atlo wloads,the crack gro wsslo wly and hydrolysis isfastenough,sothatthecrackgrows withnegligibleplasticdeformationandthefracturesurface is s mooth. These observations sho w that hydrolysis e mbrittles PET. Under develop ment for sus- tainability and healthcare are biodegradable and bio mass-derived poly mers, many of which have hydrolysablegroupsinthe mainchainsorcrosslinks.Theyareallpotentiallysusceptibletohy- drolyticcrackgrowthandembrittlement.Itishopedthatthisstudy willaidthedevelopmentand applications ofthese poly mers.