The kinetics of polycondensation and copolycondensation reactions were investigated using bis(4-hydroxybutyl) terephthalate (BHBT) and bis(2-hydroxyethyl) terephthalate (BHET) as monomers. BHBT was prepared by ester interchange reaction of dimethyl terephthalate and 1,4-butanediol. BHBT and BHET were polymerized at 270 degrees C in the presence of titanium tetrabutoxide (TBT) as a catalyst. Applying second-order kinetics for polycondensation, the rate constants of polycondensation of BHBT and BHET, k(11) and k(22), were calculated as 3.872 min(-1) and 2.238 min(-1), respectively. BHBT and BHET were also copolymerized at 270 degrees C using TBT. The rate constants of crossreactions in the copolycondensation of BHBT and BHET, k(12) and k(21), were obtained by using the results obtained from a proton nuclear magnetic resonance (H-1-NMR) spectroscopy and a high-performance liquid chromatography (HPLC). It was found that the rate constants during the copolycondensation of BHBT and BHET at 270 degrees C decreased in the order k(21) > k(11) > k(22) > k(12) and the monomer reactivity ratio of BHBT was four or five times larger than that of BHET. In calculating the crossreactions, the method by the H-1-NMR spectroscopy gave more accurate results than that by the HPLC.