A numerical study of the combustion of H2/air mixtures impinging on surfaces has been performed, using detailed chemistry and multicomponent transport, in order to elucidate the roles of surface thermal and chemical quenching in pollutant emissions. For extinguishable fuel/air mixtures, the thermal coupling with the surface has been found to be strong, and surface thermal quenching reduces NOX emissions. However, nonextinguishable mixtures are practically unaffected by changes in surface temperature, except for an increase in NO2 at low surface temperatures. It is shown that radical wall quenching can affect emissions for all flames, with H being the most important one primarily for NO2 near the surface, due to the reaction NO2 + H [LEFT RIGHT ARROW] NO + OH. A new method to elucidate the dominant paths for NO formation is also presented.