The effect of copolycondensation temperature on the monomer reactivity ratios of bis(4-hydroxybutyl) terephthalate (BHBT) and bis(2-hydroxyethyl) terephthalate (BHET) was investigated at 260, 270, and 280 degrees C, in the presence of titanium tetrabutoxide as a catalyst. Adopting 2nd order kinetics to polycondensation, the rate constants of polycondensation of BHBT and BHET, k(11) and k(22), were calculated to be 2.58, 1.30; 3.87, 2.24; and 5.29, 4.06 min(-1). In addition, the rate constants k(12) and k(21) of cross reactions could be determined as 0.91, 3.00; 1.49, 4.42; and 2.13, 5.85 min(-1) from a proton nuclear magnetic resonance spectroscopic analysis. The monomer reactivity ratios of BHBT were much larger than those of BHET, indicating the block nature of the copolycondensation, but the differences between monomer reactivity ratios were decreased with increasing polycondensation temperature, indicating that a probability of randomization was increased.