Myoglobin (Mb), in films of dimethyldidodecylammonium bromide (ddab) on graphite electrodes, is used as a catalyst to mediate the electrochemical reduction of nitrous oxide (N2O) as well as the isoelectronic ion azide (N3-) in aqueous solutions. The electrocatalytic reductions are characterized by a rate-dependent irreversibility in cyclic voltammograms of Mb/ddab in the presence of the substrates. Bulk electrolysis shows that the reduction of 15N15NO by Mb/ddab yields 15N15N as shown by GC/MS. The catalytic reduction of azide results in almost quantitative formation of ammonia. These electrocatalytic processes are rationalized as two-electron reductions, with the catalyst cycling between the Fe(I) and Fe(III) states of Mb. To our knowledge, this is the first characterization of N2O reduction by an Fe porphyrin or heme protein.