Abstract The catalytic decomposition of H 2O 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 H 2 by O 2 and N 2O. The effects of added H 2, D 2 and O 2 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 NH 3 over the same catalyst. However, no NH or NH 2 radicals were detected in gaseous decomposition products up to 1100 K.