The reduction of NO3- on palladium/copper electrodes has been studied using differential electrochemical mass spectroscopy (DEMS), rotating ring-disk electrodes (RRDE) and quartz microbalance electrodes (ECQM). In acidic electrolytes, the activity increases linearly with Cu coverage, in alkaline electrolytes, a different dependence on coverage is observed. One monolayer of Cu gives a different selectivity from bulk copper. The adsorption of NO3- is competitive with SO42-, whereas Cl- adsorption blocks the reduction. Competitive adsorption lowers both the activity and the selectivity to N-2. Copper activates the first electron transfer, the role of palladium is to steer the selectivity towards N-2. The trends in activity and selectivity are explained in terms of coverage of N-species.