The kinetics of the forward polycondensation reaction of polybutylene terephthalate (PBT) has been investigated using thermogravimetric analysis (TGA). PBT - prepolymer with an initial degree of polymerization of 5.5 was used as starting material. The PBT prepolymer was prepared from dimethyl terephthalate (DMT), and 1,4 - butanediol with 470 ppm wt (with respect to DMT) of tetrabutoxytitanate as catalyst. The process conditions in the TGA were such that the weight loss curves obtained from the TGA could be correlated uniquely and directly to the forward polycondensation reaction kinetics due to the absence of (1) mass transfer limitations in the removal of 1,4 butanediol from the melt, and (2) degradation reactions. It is shown that in the temperature range of 230-270degreesC, the forward polycondensation reaction can be expressed using second-order kinetics in terms of the hydroxyl end group concentration of the PBT polymer. (C) 2005 American Institute of Chemical Engineers.