Blends of polyhydroxybutyrate and its copolymers with hydroxyvalerate have been prepared. Under certain conditions of composition and temperature these blends can phase separate. It is shown that for freshly prepared samples, crystallization from a biphasic, rather than a homogeneous, melt leads to improvement in yield and fracture properties. However, during storage at room temperature, an ageing process occurs and the improvement in properties is largely lost.