Carboxylic acid end groups of poly(butylene terephthalate) (PBT) granules were modified with an excess of 1,2-epoxybutane in supercrit. CO2 and in supercrit. and subcrit. mixts. of CO2 with 10 mol% 1,4-dioxane. Temps. of 120 and 180 Deg and pressures of 60, 150 and 300 bar were applied. Best results were obtained for the modification in scCO2 contg. 10 mol% 1,4-dioxane at 180 DegC. The carboxylic acid end group concn. was reduced from 44 mmol/kg to approx. 10 mmol/kg, which resulted in a significantly enhanced hydrolytic stability. This was demonstrated by measuring, both for unmodified and for 1,2-epoxybutane blocked PBT, the intrinsic viscosity as a function of hydrolysis time as well as the no. and wt. av. mol. wts. (Mn and Mw) before and after hydrolysis. After the modification, some PBT granules appeared more brittle than the unmodified PBT granules. The crystallinity proved to be enhanced by the plasticizing effect of CO2/1,4-dioxane at 180 Deg. No indications were obtained that the 1,2-epoxybutane modified end groups exhibit a nucleating effect on the PBT crystn.