The selective hydrogenation of para- chloronitrobenzene was investigated over P-2W and P-1 nickel borides. The effects of solvents, additives and reaction conditions were examined. P-2W nickel boride was found to be more active than P-1 nickel boride and the commercial catalyst of Raney nickel for the reduction of the nitro-group. The solvent not only affected the rate but also the dehalogenation and the formation of intermediates. The nickel borides were more active in the protic solvents than in the non-protic solvents according to the following order : ethanol > methanol > cyclohexane > methylacetate. Only a small amount of intermediate (azoxychlorobenzene) and dehalogenation product (aniline) formed in the protic solvents; however, these products did not form in the non-protic solvents. The addition of molybdenum effectively promoted the reduction of the nitro-group and repressed dehalogenation. This reaction was a structure-sensitive reaction, with its rate decreasing with the dispersion of boride catalysts. The reaction rate was expressed as r=k(o)e(-Ea/RT).C(R).P(H)-1/2 at a lower concentration and r = constant at a higher concentration.