The oxidation of formic and acetic acids with hydroxyl radicals was studied as a model for the oxidation of larger carboxylic acids using first principles calculations. For formic acid, the CBS-QB3 activation barriers of 14.1 and 12.4 kJ/mol for the acid and for the formyl channel, respectively, are within 3 kJ/mol of benchmark W1U values. Tunneling significantly enhances the rate coefficient for the acid channel and is responsible for the dominance of the acid channel at 298 K At 298 K, tunneling correction factors of 339 and 2.0 were calculated for the acid and the formyl channel using the small-curvature tunneling method and the CBS-QB3 potential energy surface. The Wigner, Eckart, and zero-curvature tunneling methods severely underestimate the importance of tunneling for the acid channel. The resulting reaction rate coefficient of 0.98 x 10(5) m(3)/(mol . s) at 298 K is within a factor 2-3 of experimental values. For acetic acid, an activation barrier of 11.0 kJ/mol and a tunneling correction factor of 199 were calculated for the acid channel. Two mechanisms compete for hydrogen abstraction at the methyl group, with activation barriers of 11.9 and 12.5 kJ/mol and tunneling correction factors of 9.1 and 4.1 at 298 K. The resulting rate coefficient of 1.2 x 10(5) m(3)/(mol . s) at 298 K and branching ratio of 94% compare well with experimental data.