Biodegradability of several homo- and copolyesters, containing tetrahydropyran rings in their backbones with or without pendant groups, was investigated by degradation tests, both in soil and in an activated sludge. These polyesters were hydrolytically degraded to lower molecular weight compounds, and eventually to hydroxytetrahydropyran carboxylic acids, at different rates, depending on their molecular structure. Quantitative determination of carbon dioxide, generated during the treatment with the activated sludge, showed that the hydrolysates from polyesters 2 and 10, without pendant alkoxycarbonyl groups, were catabolized by microorganisms. It was concluded from these results that at least the polyesters without pendant groups were biodegradable, and that polyester 10, consisting of 2,6-linked tetrahydropyran rinp, underwent biodegradation more readily than polyester 2, consisting of 2,5-linked tetrahydropyran rings.