Palladium catalysts on various supports were studied for the hydrogenation of aqueous nitrite solution. The effects of reaction parameters such as nitrite and hydrogen concentration, pH and reaction temperature on the selectivity were discussed. Internal concentration gradients are influencing the selectivity significantly. Gradients of protons and nitrite are responsible for increasing ammonia selectivity. Carbon based catalysts were more selective towards ammonia as compared to oxidic supports. We propose that electron conductivity of carbon supports influences internal concentration gradients of both hydrogen and protons, favouring ammonia selectivity. (C) 2010 Elsevier B.V. All rights reserved.