Hydroxyl radical desorption in the heterogeneous catalytic reactions of water or hydrogen with oxygen was examined by laser-induced fluorescence spectroscopy. The catalytic activities of Pt, Al2O3, and basic metal oxides (MgO, CaO, SrO, BaO) supported on Al2O3 were studied in the pressure range 0.1-10 Torr and the temperature range 1100-1300 K. In the case of OH generation from water, the catalytic activities of Pt and all oxides except Al2O3 were very high and the OH concentration reached the equilibrium value within a residence time of 4 ms. In the case of hydrogen oxidation, differences in the catalytic behavior were clearly observed. The surface reaction mechanisms and the effects of Al2O3 and MgO on gas-phase ignition are discussed.