Downstream processing in the biorefinery is often faced with multicomponent process streams containing carboxylic acids and alcohols. Isolation of carboxylic acids may be simplified by direct esterification with the alcohol present in the process stream. However, uncatalyzed esterification is generally slow at moderate temperature. In multicomponent mixtures, though, stronger carboxylic acids act as catalysts for the esterification of weaker carboxylic acids. Single-acid esterification of formic acid, acetic acid, and propionic acid with methanol were investigated, and a kinetic model was developed considering the intramolecular catalytic effect through acid dissociation. With the kinetic constants derived from single-acid esterification, ester formation in multi-acid esterification was predicted and verified experimentally with a maximum sum of mean square error of 8.70 X 10(-4). In multi-acid esterification, the catalytic influence of formic acid on acetic acid and propionic acid esterification was shown. Due to higher equilibrium constants transesterification of methyl formate to methyl acetate and methyl propionate, respectively, is observed and predicted by the model.