The kinetics of the bulk copolyesterification at 205 degrees C between o-phthalic anhydride, oleic acid, and neopentyl glycol in a nonequimolecular ratio, ([-COOH]/[-OH]) = 0.7, and in the absence of an external catalyst has been investigated. From a practical point of view and to carry out the kinetic analysis of the present system, we have taken into account the following considerations : (i) After the initial mixing of reagents, the ring-opening reaction of the o-phthalic anhydride by a hydroxyl group is instantaneous. (ii) This is also the case even with two hydroxyl groups from the same neopentyl glycol molecule; that is they probably have almost the same reactivity toward the anhydride ring. (iii) The reaction of a carboxylic group originated according to consideration i with a second neopentyl glycol molecule is equally instantaneous, i.e.,the reaction between one o-phthalic anhydride molecule and two neopentyl glycol molecules. (iv) Also, after the initial mixing, the reaction of a first molecule and a second molecule of oleic acid with a neopentyl glycol molecule to give an inactive compound is also instantaneous. The results establish that the overall kinetic order of the copolyesterification is third order. The copolyesterification is three-halves order in carboxylic and three-halves in hydroxyl group concentrations. A chemical kinetic model was developed for the copolyesterification of o-phthalic anhydride, oleic acid, and neopentyl glycol. The mechanism was based on very simple elemental reactions. All relevant reactions were considered, and a final selection of 12 reactions was made. The proposed 12-step reaction mechanism gives good agreement between experimental results and computer simulations.