A preliminary study of the calorimetric response of highly crystalline samples of microbial poly(hydroxybutyrate) (PHB) has been carried out using a new temperature-modulated differential scanning calorimetry (MDSC(TM)). This instrument extends thermal analysis to a differentiation between reversible and non-reversible processes. The thermograms clearly show the reversible glass transition of PHB, even for unquenched samples. The presence of non-reversible cold crystallization, furthermore, enhances the other recrystallization phenomenon which is peculiar to the rearrangement of the crystalline regions during the melting process. It is anticipated that a through theoretical analysis of the thermal response using this approach will lead to new understanding of phase transfers during kinetic processes.