A simple two-parameter model was proposed for the kinetics of polyesterification reactions. The kinetic model is based on the true reaction mechanism, i.e. on the shift of the ionic equilibria in the carboxylic acid (RCOOH) protolysis during the reaction, and on the bimolecular nucleophilic substitution of the protolysed acid with the alcohol (R'OH). The model describes the increase of the reaction order with respect to the carboxylic acid from 1 to 2 as the esterification proceeds. The rate equation r = kc(COOH)(n)c(OH) was used, where n is a function of the C carboxylic acid concentration. The kinetic model was tested with the classical data of Flory obtained for diethylene glycol-adipic acid and the lauryl alcohol-adipic acid reactions. The model provided an excellent description of the data over the entire range of conversions of the carboxylic acids and it can be extended to new polyesterification systems. (C) 2004 Elsevier B.V. All rights reserved.