The catalytic reduction of nitrite in water with hydrogen has been studied using a new strategy to control selectivity. The catalysts used are based on size-controlled Pd-AOT nanoparticles, synthesized via sodium bis[2-ethylhexyl] sulfosuccinate (AOT)/isooctane reverse microemulsion, supported on activated carbon. The most remarkable feature of the catalysts is the negligible selectivity toward ammonium, which is attributed to shielding of Pd NPs by AOT and blockage of the active centers related to ammonium generation. The shielding by AOT also reduces the activity of Pd-AOT NPs, which can be overcome by immobilization on carbon and thermal treatment at mild conditions (473 K, N2 atmosphere), although excessive removal of AOT results in higher ammonium production. Complete nitrite conversion with total selectivity to N2 was achieved at room temperature for the Pd-AOT/C catalysts at controlled pH media using CO2 as buffer agent. Moreover, the catalytic activity results at controlled pH show that the nitrite reduction reaction is not structure sensitive.