The mechanism of the electrochemical reduction of nitric oxide (NO) by hemin adsorbed at pyrolitic graphite was investigated. The selectivity of NO reduction was probed by combining the rotating ring disk electrode (RRDE) technique with a newly developed technique called on-line electrochemical mass spectroscopy (OLEMS). These techniques show that NO reduction by adsorbed heme groups results in production of hydroxylamine (NH2OH) with almost 100% selectivity at low potentials. Small amounts of nitrous oxide (N2O) were only observed at higher potentials. The rate-determining step in NO reduction most likely consists of an electrochemical equilibrium involving a proton transfer, as can be derived from the Tafel slope value of 62 mV/dec and the pH dependence of -42 mV/pH. The almost 100% selectivity toward NH2OH distinguishes this system both from NO reduction on bare metal electrodes, which often yields NH3, and from biological NO reduction in cytochrome P450nor, which yields N2O exclusively.