The autothermal behavior of H-2/O-2 mixtures over a platinum foil catalyst is investigated experimentally and theoretically. Experiments were conducted at atmospheric conditions, at levels of nitrogen dilution ranging between 80% and 92% by volume. The stagnation flow geometry was modeled using simplified multicomponent transport and detailed reaction mechanisms, both in the gas phase and on the catalyst surface. A maximum autothermal temperature was found at the unexpected, nonstoichismetric, H-2/O-2 volume ratio of approximately unity. Experimental and model results for the effect of dilution on autothermal temperatures and the flammability limits are presented and discussed. Sensitivity analyses indicate that the upper flammability limit depends on the relative adsorption rates of H-2 and O-2 onto the platinum surface, and the desorption of adsorbed H*, while the location of the maximum in autothermal temperature depends on the transport of H-2 and O-2. It is shown that repulsive H*-H* interactions on the surface may be essential for accurate prediction of the upper flammability limit.