Solid acid catalysts are attractive replacements for liquid mineral acids for esterification reactions, where ester products are important for end products that are as diverse as perfumes and biofuels. Although the mechanism of homogeneous esterification has long been known, the mechanistic pathways for esterification over heterogeneous catalysts are still being pursued. This paper presents the results of an investigation into the mechanism of gas-phase esterification in the temperature range of 90-140 degrees C for a simple carboxylic acid (acetic acid) and short-chain alcohols (methanol and ethanol) that have been catalyzed by a solid catalyst with only Bronsted acid sites (silica-supported Nafion, SAC- 13). In this study, evidence from pyridine poisoning experiments suggests that the esterification proceeds via a single site mechanism on solid Bronsted acid catalysts such as SAC-13, as has also been concluded for reaction in the liquid phase at temperatures of :560 degrees C. Based on initial rate data for methanol reaction, the order of reaction for acetic acid shifted only from 0.7 to 0.9 as the temperature changed from 90 degrees C to 130 degrees C. However, the order of reaction for methanol shifted from 0.2 to -0.9 for the same temperature range. The results for the ethanol reaction were similar. Based on these outcomes, the reaction most probably happens between adsorbed acetic acid molecules on the active sites of the catalyst and alcohol molecules from the bulk fluid. However, as the temperature increases from 90 degrees C to 130 degrees C, the rate-limiting step becomes acetic acid adsorption. The reaction is also hindered by the competitive adsorption of the alcohol on the acid sites of the catalyst. Experiments that involve the preadsorption of reactants offered further evidence of the inhibiting effect of the alcohol and the change in the rate-controlling step for reaction at temperatures of > 100 degrees C.