Catalytic ammonia denox reactions over a polycrystalline Pt surface in the temperature range 800-1330 K have been investigated by laser-induced-flourescence detection of the OH radical produced on and subsequently desorbed from the surface. For the NO2-NH3 system, strong curvature is observed in the low temperature region of the Arrhenius plots, which could be attributed to the presence of different desorption processes. The two extracted apparent OH desorption energies increase from 30 to 49 kcal/mol and from a surprisingly low value of 2 to 27 kcal/mol as the NO2NH3 reactant mixture ratio is decreased from 70 to 0.037. On the other hand, for the NO-NH3 system, no such curvature in the Arrhenius plots is observed and the OH desorption energy decreases from 38 to 26 kcal/mol as the NO/NH3 ratio is decreased from 70 to 0.4. The different trends in the apparent OH desorption energies as a function of the NOx/NH3 ratio in the present two systems can be rationalized by the degree of coverage of chemisorbed O atoms relative to that in the earlier studied O2-NH3 system.