The effect of initial solution apparent pH (pH.) on nano-sized nickel in the catalytic hydrogenation of p-nitrophenol to p-aminophenol was investigated in detail. Experimental results show that the catalytic activity of the nano-sized nickel shows a strong dependence on the initial Solution pH., and is the highest at the initial solution pH(a) 4.8. At lower or higher pH(a) values the nano-sized nickel will obviously deactivate, but the deactivation mechanisms are fully different. With respect to the former, the nickel dissolution and the strong adsorption of the complex compound of nickel ion and p-nitrophenol on the surface of nano-sized nickel are the main reasons. The severe agglomeration of nickel particles also causes the deactivation of nickel used at the initial solution pH(a) 1.6. For the latter the main reasons are the formation of nickel hydroxide surface species and the decrease of p-nitrophenol adsorption on the hydrogenation sites. (c) 2008 Published by Elsevier B.V.