Isothermal crystallization and morphology of biodegradable PBST copolyesters with high content of BT units were investigated. The crystallization kinetic analysis showed that the Avrami exponent n ranged from 3.2 to 4.9, indicating a three-dimensional growth initiated by homogeneous nucleation mechanism at low crystallization temperature and heterogeneous nucleation mechanism at high temperature. After isothermal crystallization, multiple melting peaks were observed during DSC heating, and were explained by the melting and recrystallization model. The equilibrium melting temperatures of the PBST copolyesters were determined by Hoffman Weeks and Gibbs Thomson methods, respectively. Two methods, however, gave different values for each sample. The origin of the complexity and its influences on the equilibrium melting temperature of the copolyesters were analyzed. According to WAXD measurement, the copolyesters were identified to have the same crystal structure as that of PBT, suggesting that only the BT units were crystallized in the system whereas the BS units were in the amorphous state. POM micrographs showed that the crystals of the copolyesters were composed of little spherulites, beneficial to forming flexible fibers. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 623-630, 2010