The mechanism of the heterogeneous hydrogenation of nitrite over a Pt/Al2O3 catalyst layer deposited on a ZnSe internal reflection element was investigated in water using attenuated total reflection infrared spectroscopy. In addition to adsorbed nitrite, hydrogenation intermediates NO(ads), “HNO”(ads), and HNO2−(ads) are formed on the platinum surface. Hydrogenation of the surface intermediates mainly results in NH4+, but also traces of N2O are observed as well, which is believed to be an intermediate in the formation of nitrogen. “HNO”(ads) is the most prominent surface species during steady state operation and is therefore involved in the rate-determining step. Some NO(ads) accumulates at steps in transient experiments, showing a very low reactivity toward N2O. The results show that although the reaction pathways of nitrite hydrogenation on platinum and palladium are rather similar; the rate-determining steps on the metals are clearly different.