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 270ยฐC 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, kll and kZ2, were calculated as 3.872 min-I and 2.238 min-', respectively. BHBT and BHET were also copolymerized at 270ยฐC using TBT. The rate constants of crossreactions in the copolycondensation of BHBT and BHET, k12 and k2,, were obtained by using the results obtained from a proton nuclear magnetic resonance ('H-NMR) spectroscopy and a high-performance liquid chromatography (HPLC). It was found that the rate constants during the copolycondensation of BHBT and BHET at 270ยฐC decreased in the order kzl > k l , > kT2 > k12 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-NMR spectroscopy gave more accurate results than that by the HPLC.