Ab initio molecular orbital theory has been used in the study of the reaction of OH radicals with CH3C(O)H and CF3C(O)H. Equilibrium geometries and transition-state structures have been fully optimized at the UHF/6-31G* and UMP2/6-31G* levels of theory. Activation energy barriers and heats of reaction have been estimated using fourth-order Møller–Plesset perturbation theory with spin-projection, including single, double, triple and quadruple excitations [PMP4(SDTQ)]. Results for the reaction of OH radicals with CH3C(O)H and CF3C(O)H are compared with available experimental data.