Boron-doped diamond films were used as electrodes for the reduction of nitrate and nitrite ions to ammonia. Prior to the electrochemical investigation, some in-depth analysis of the diamond films, was undertaken using various techniques. It was found that the plasma-assisted chemical vapor deposition process is superior to the chemical deposition process in controlling the (boron) doping process. The analysis also indicates that some of the boron is occluded in the film along the channels (grain boundaries) between the diamond crystallites. The faradaic efficiency (FE) of reduction of the nitrite (nitrate) to ammonia was found to be greater than unity and could be partially accounted for by the dissolution of the Si substrate itself. Further experiments with free-standing diamond films showed a rather smaller FE; nevertheless it was still greater than unity. A submonolayer of bound nitrogen was found on the diamond surface after the electrolysis. The present results are attributed to a catalytic mechanism for the multi-electron process described here.