The miscibility of poly(D-(-)-3-hydroxybutyrate)/atactic poly(methyl methacrylate) (PHB/aPMMA) blends has been investigated as a function of blend composition and thermal treatments. The DSC thermograms obtained by heating up samples of PHB/aPMMA blends quenched from 200 and from 185 degrees C to liquid nitrogen temperature show the presence of a single glass transition temperature, whereas the thermograms of blends annealed at temperatures selected in the range 140-170 degrees C display the presence of two glass transition temperatures. The SEM analysis of the fractured surfaces of the samples show that the quenched blends are homogeneous, whereas two phases are present in the annealed blends. The results have been accounted for by assuming an upper critical solution temperature (ucst). At T > ucst, the PHB and aPMMA components are completely miscible at all compositions, whereas at lower temperature the melt phase separates in the two pure components. The glass transition behavior observed for the quenched blends has been interpreted by using the free volume theory presented by Kovacs. This theory has been shown to describe the experimental T-g values as a function of blend composition if the interaction term g is assumed negative.