Carboxylic acid end groups of poly(butylene terephalate) (PBT) granules were modified with an excess of 1,2-epoxybutane in supercritical CO2 and in supercritical and subcritical mixtures of CO2 with 10 mol% 1,4-dioxane. Temperatures of 120 and 180 degreesC and pressures of 60, 150 and 300 bar were applied. Best results were obtained for the modification in SCCO2 containing 10 mol% 1,4-dioxane at 180 degreesC. The carboxylic acid end group concentration was reduced from 44 mmol/kg to approximately 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 number and weight average molecular weights ((M) over bar (n) and (m) over bar (w)) 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 plasticising effect of CO2/1,4-dioxane at 180 degreesC. No indications were obtained that the 1,2-epoxybutane modified end groups exhibit a nucleating effect on the PBT crystallisation. (C) 2003 Elsevier Science Ltd. All rights reserved.