The catalytic decomposition of H2O over a polycrystalline platinum surface has been studied in a low pressure flow system. Hydroxyl radicals were detected in gaseous decomposition products above about 820 K by laser-induced fluorescence at 300 nm. The activation energy for the HO desorption process, HO* → HO(g)+*, was determined to be 30 ± 1 kcal/mol, which is in excellent agreement with the values reported previously for the Pt-catalyzed oxidation of H2 by O2 and N2O. The effects of added H2, D2 and O2 on HO formation were also investigated and the observed data can be satisfactorily accounted for by a mechanism which involves chemically adsorbed H, O and HO species. A similar but brief experiment was carried out for the catalytic decomposition of NH3 over the same catalyst. However, no NH or NH2 radicals were detected in gaseous decomposition products up to 1100 K.