We present ab initio multiconfiguration self consistent field calculations, using a polarized basis set, of equilibrium and transition-state structures relevant to the two competing mechanisms for the unimolecular decomposition of acetic acid. Single-point calculations using a modified Gaussian-2 method indicate that the lowest energy decomposition pathway for acetic acid is decarboxylation through a four-center transition state leading to methane and carbon dioxide, with a barrier of 71.8 kcal/mol. Dehydration, leading to ketene and water, has a calculated overall activation energy of 73.1 kcal/mol and occurs most easily through a two-step mechanism : 1,3 hydrogen migration to an enediol followed by elimination of water via a four-center transition state. The direct (one-step) dehydration of acetic acid has a calculated barrier of 76.4 kcal/mol. Transition state theory calculations indicate that all three decomposition routes are competitive at high temperatures : the predicted branching between decarboxylation, one-step dehydration, and two-step dehydration is 1.00:0.34:0.28 at 900 K and 1.00:1.11:0.31 at 1500 K. The hydration of ketene to acetic acid in the gas phase is predicted to occur competitively at high temperature by addition of water to the CO pi-bond followed by a 1,3 hydrogen migration and by one-step addition to the CC pi-bsnd.