In this study, thymine and melamine were introduced as nucleating agents for poly(3-hydroxybutyrate-co-3-hydroxyvalerates) (PHBVs) and poly(3-hydroxybutyrate) (PHB), and their effects were compared with that of boron nitride (BN). Because the overall crystallization rate of PHBVs decreases significantly with the increase in the 3-hydroxyvalerate comonomer content, the study focused on the crystallization of PHBVs. Isothermal crystallization kinetics of the neat PHBVs and the nucleated PHBVs were studied by differential scanning calorimetry (DSC). The Avrami equation was derived and the parameters were assessed for the nucleation and crystal growth mechanism. The nucleation and crystal growth were examined using polarized optical microscopy All nucleating agents had similar particle sizes and showed good dispersion in the polymer matrix, as revealed by scanning electron microscopy. The results indicated that BN and thymine significantly increased the overall crystallization rate for all PHBVs studied and demonstrated very similar nucleating effects. Melamine reacted with PHBVs and accelerated the thermal degradation, and hence was less effective in nucleating PHBVs. (c) 2007 Wiley Periodicals, Inc.