We have elucidated the mechanism for ethyl oleate hydrolysis in high temperature Water and its reverse reaction, oleic acid esterification in near- and supercritical ethanol in the absence of any other added compounds. Both reactions are add catalyzed. H+ (from dissociation of water and oleic acid) and oleic acid serve as catalysts for hydrolysis, and H+ alone is the catalyst for esterification. The rate equation arising from the proposed mechanism provided a good fit of experimental conversion data for both. hydrolysis and esterification: The rate equation accurately, predicted the influence of pH on hydrolysis for acidic and near-neutral conditions. The mechanistic model exhibits the ability to make quantitatively accurate predictions Within and outside the original parameter space, especially for a multicomponent system. Sensitivity analysis shows that the values of the dissociation constant of oleic acid. in ethanol, water, and ethanol-water systems strongly influence the predicted conversions. There is a need for experimental measurement of pK(a) for fatty acids in both water and alcohols at elevated temperatures.