Polymorphic crystals and complex multiple melting behavior in an aliphatic biodegradable polyester, poly(butylene adipate) (PBA), were thoroughly examined by wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). Further clarification on mechanisms of multiple melting peaks related to polymorphic crystal forms in PBA was attempted. More stable a-form crystal is normally favored for crystallization from melt at higher temperatures (31-35 degrees C), or upon slow cooling from the melt; while the beta-form. is the favored species for crystallization at low temperatures (25-28 degrees C). We further proved that PBA crystallization could also result in all alpha-form even at low temperatures (25-28 degrees C) if it crystallized with the presence of prior -form nuclei. PBA packed with both crystal forms could display as many as four melting peaks (P1 similar to P4, in ascending temperature order). However, PBA initially containing only the alpha-crystal exhibited dual melting peaks of P1 and P3, which are attributed to dual lamellar distributions of the alpha-crystal. By contrast, PBA initially containing only the beta-crystal could also exhibit dual melting peaks (P2 and P4) upon scanning. While P2 is clearly associated with melting of the initial beta-crystal, the fourth melting peak (P4), appearing rather broad, was determined to be associated with superimposed thermal events of crystal transformation from beta- to alpha-crystal and final re-melting of the new re-organized alpha-crystal. Crystal transformation from one to the other or vice versa, lamellae thickening, annealing at molten state, and influence on crystal polymorphism in PBA were analyzed. Relationships and mechanisms of dual peaks for isolate alpha- or beta-crystals in PBA are discussed. (c) 2005 Wiley Periodicals, Inc.