The electrochemical reduction of nitrous oxide on iridium single-crystal electrodes constitutes a paradigmatic example of sensitivity to the surface structure. Ir(111) and Ir(110) electrodes catalyze the process, while Ir(100) is almost totally inactive. An electroreduction mechanism is proposed consisting of the initial dissociative adsorption of N2O Yielding oxygen adatoms followed by the reduction of the oxygen adatoms by electrosorbed hydrogen (rate-determining step). The proposed mechanism explains the main features of the voltammetric reduction waves on the active basal planes and provides an explanation for the lack of activity of the Ir(100) electrode. The latter is consistent with the observation that the adsorption/desorption of hydrogen occurs as a sharp phase transition (theta(H) = 0 <----> theta(H) = 1), the N2O reaction being quenched at lower potentials.