Synthesis of unsaturated polyesters using adipic acid, ethylene glycol, and fumaric acid in the absence and presence of a foreign acid (phosphoric acid) as catalyst was carried out by a two-stage method under constant reaction temperatures of 160-180 degrees C and at different ratios of diol to diacid. The experimental data fit the Chen-Wu rate equations for self-catalyzed and acid-catalyzed reactions. The reaction rate constants and activation energies for both systems were calculated. The rate constants of fumaric acid-ethylene glycol systems were found to be nearly constant and had negligible variations with increasing chain length of polymer.