Microbially produced poly(3-hydroxybutyrate) (P(3HB)) and a copolymer were investigated by measuring tensile strength, Young’s modulus and molar mass after different periods of thermal treatment at temperatures slightly above their melting temperatures in order to describe the changes of molecular and mechanical properties. The tensile strength turns out to be strongly influenced by time and temperature of heat treatment. After a heat treatment for 10 min at 190 degrees-C the tensile strength drops to 30% of its initial value.