A comprehensive kinetic investigation of the esterification of acetic acid with methanol in both the liquid phase (21-60 degrees C) and the gas phase (100-140 degrees C) was carried out using tungstated zirconia (WZ) as the catalyst. The goal of this study was to derive rate law expressions and to propose a reaction mechanism that would support reaction rate data for esterification on WZ. Upon increasing the concentration of acetic acid, an increase in the rate of esterification was obtained at all reaction temperatures. In contrast, the reaction order with respect to methanol evolved from positive to negative as the reaction temperature increased. Using a model discrimination procedure, we found that a single-site (Eley-Rideal) mechanism with the adsorbed carboxylic acid reacting with the methanol from the gas/liquid phase successfully describes these reactions. One important conclusion of this study was that, even though there were significant differences in the power law exponents for gas- and liquid-phase esterifications, the same reaction mechanism can successfully describe both situations. We propose that the adsorption of the carboxylic acid becomes the rate-determining step (RDS) as the reaction temperature increases. At lower esterification temperatures (liquid phase), the rate-determining step appears to be the nucleophilic attack of the alcohol on the adsorbed/protonated acetic acid molecules. At higher reaction temperature (gas phase), the adsorption of the carboxylic acid becomes rate determining. It was also found that the catalytic activity of WZ was inhibited by water similarly to other strong acid catalysts.