The rate of decarboxylation of 2,4-dimethoxybenzoic acid (1) is accelerated in parallel to the extent that the carboxyl group acquires a second proton (1H(+)). However, the conjugate acid would resist C-C bond breaking as that would lead to formation of doubly protonated CO2. An alternative via formation of a higher-energy protonated phenyl tautomer (2H(+)) prior to C-C bond breaking would produce protonated CO2, an energetically inaccessible species that can be avoided by transfer of the carboxyl proton to water in the same step. Headspace sampling of CO2 that evolves in the acid-catalyzed process and analysis by GC-IRMS gives a smaller than expected value of 1.022 for the carbon kinetic isotope (CKIE), k(12)/k(13). While this value establishes that C-C cleavage is part of the rate-determining process, intrinsic CKIEs for decarboxylation reactions are typically greater than 1.03. Computational analysis of the C-C bond cleavage from 2H(+) gives an intrinsic CKIE of 1.051 and suggests two partially rate-determining steps in the decarboxylation of 1: transfer of the second carboxyl proton to the adjacent phenyl carbon and C-C cleavage in which the carboxyl proton is also transferred to water. Applying the principle of microscopic reversibility to fixation of CO2 in acidic solutions reveals the importance of proton transfers to both carbon and oxygen in the overall fixation process.