The bimolecular thermal reactions of carboxylic acids were studied using quantum mechanical molecular modeling. Previous work1 investigated the unimolecular decomposition of a variety of organic acids, including saturated, alpha,beta-unsaturated, and beta,gamma-unsaturated acids, and showed that the type and position of the unsaturation resulted in unique branching ratios between dehydration and decarboxylation, [H2O]/[CO2]. In this work, the effect of bimolecular chemistry (wateracid and acidacid) is considered with a representative of each acid class. In both cases, the strained 4-centered, unimolecular transition state, typical of most organic acids, is opened up to 6- or 8-centered bimolecular geometries. These larger structures lead to a reduction in the barrier heights (2045%) of the thermal decomposition pathways for organic acids and an increase in the decomposition kinetics. In some cases, they even cause a shift in the branching ratio of the corresponding product slates.