Platinized platinum catalysts were modified "in situ" by copper (sub)monolayers deposited under controlled conditions by electrochemical methods. They were characterized "in situ" by linear potential sweep cyclic voltammetry. Various Cu-ads/Pt catalysts with different copper coverages (theta(Cu)) were obtained and tested for the reduction of nitrate and nitrite in aqueous solutions. The intrinsic catalytic activity (at t = 0) and the selectivity in ammonia of Cuads/Pt depend on the platinum surface fraction occupied by copper adatoms (theta(Cu)). The highest activities and selectivities in ammonia are obtained with theta(Cu) approximate to 0.4-0.6. Moreover, higher activities were obtained in the reduction of nitrate than in the reduction of nitrite. The change of catalysts potential, measured during the reaction, arises from the change of the solution pH, which is due to the generation of OH- according to the global reduction reactions. The spontaneously established catalyst potential is a mixed potential corresponding to the different oxidoreduction reactions involved.