The direct-esterification reaction of succinic acid (SA) with 1,4-butanediol (BD) was investigated in the temperature range from 170 degrees C to 190 degrees C with and without a monobutyl tinoxide (MBTO) catalyst. The extent of reaction was measured by the quantity of water distilled from the reaction. The effects of reaction temperature, molar ratio of SA and ED, and MBTO concentration on the kinetics were studied. The experimental kinetics results for uncatalyzed reactions agreed quite well with the Flory model. A new complex model was developed for the MBTO catalyzed reaction, and the experimental results showed good agreement with the complex model.