The kinetic equation of polyesterification proposed by Tang is discussed from the view point of the medium effect on the ionization of weak acids，so that the ionization constant，Ke，of the reacting acid would not be a constant during the course of poly-condensation and consequently the 2 1/2 order kinetic equation would not be valid．Ex-perimental data show that Ke is actually a function of the extent of reaction．No ap-preciable conductivity is observed when the extent of reaction is over 0.65．Both the kinetic data of the present study and those of Flory for the polyesterification of ethy-lene glycol and adipic acid do not exactly follow the 2 1/2 order kinetic equation．