Rate constants and kinetic isotope effects for the 1,2-H shift of methoxyl and benzyloxyl radicals were studied in the presence of water molecules. The electronic structure calculations were-carried out at the UB3LYP/6-31G* level, and the dynamics calculations were performed using the variational transition state theory with semiclassical multidimensional corrections for tunneling. The study deals with 1:1 and 1:2 radical-water complexes in the gas phase. It was found that water catalyzes these rearrangement reactions by forming a bridge containing two water molecules. The dynamics calculations show that the methoxyl-water complexes react only very slowly. The rate constants for 1:2 complexes with the benzyloxyl radical are in relative good agreement with the results of laser flash photolysis. The kinetic isotope effects calculated using heavy water indicate that tunneling makes an important contribution in the 1:1 complex, whereas the contribution due to vibrations is more important for the 1:2 complexes. In both cases, the kinetic isotope effects are substantial. It is concluded that the 1,2-H shift for both radicals is catalyzed by two water molecules through a mechanism that involves the formation of a preliminary 1:1 complex.