AbstractThe overall third‐order transesterification reaction, comprising one order each with respect to hydroxyl group, ester group, and catalyst, was considered on assuming a varying reactivity of OH group. The integrated expression for the catalyzed (constant catalyst concentration) reaction predicts an apparent order of reaction n dependent on the extent of reaction p: for p → 0, n = 2; whereas p → 1, n = 5/4. For practical reasons the order of reaction was redefined in terms of (1 − p) ∝ P, (where Pn is the number‐average degree of polycondensation). The integrated expression was compared with published experimental results of diphenyl carbonate transesterification by 4,4,‐isopropylidene diphenol. At T < 200°C, the expression was well obeyed, at 200 < T < 230°C the reaction showed an intermediate behavior, whereas at still higher temperatures the third‐order reaction was found. It is proposed that these changes are due to variations in the chemical nature of the zinc catalyst, resulting in the difference in the transesterification mechanism. A convenient approximate relation for calculating Pn from measured viscosity‐average degree of polymerization Pv is also proposed.