The reaction of water and oxygen to form hydroxyl radicals over the metal oxide catalysts La2O3, Nd2O3, Sm2O3, Yb2O3, CeO2, and MgO was studied al pressures up to several Torr. After reaction over 27 mg of La2O3 at 900 degrees C, the measured concentration of hydroxyl radicals in the gas phase, detected by laser induced fluorescence spectroscopy, was equivalent to the expected equilibrium concentration. The reaction becomes kinetically controlled at catalyst loadings below 5 mg. Oxygen incorporation at the surface sites may be the rate limiting step in the catalytic cycle. The activities of the catalysts decrease from La2O3, the most active, to CeO2, which is inactive under these reaction conditions. This order is the same as that found for methyl radical formation over these oxides, suggesting that the active site on the catalyst surface is the same for both hydroxyl radical formation and methyl radical formation.