A complex mathematical model accounting for the hydrogen dissolution process in suspensions and mass-transfer steps at the Liquid-solid interface for the gas and Liquid components is given. The calculated data according to the model for the reaction A --> B --> C shows, that the yield of an intermediate product B is very much affected by the relation of the gas component mass-transfer coefficient on the gas-liquid interface to that on the liquid-solid one. The hydrogenation of chlornitroaromatic compounds was analysed. The kinetics of the catalytic reduction of p-chlornitrobenzene to p-chloraniline via corresponding arylhydroxilamine on the Ir/C catalyst experimentally in a batch reactor has been studied. In this process the first reactions depend on the hydrogen concentration but the second ones are not dependent - this is a disproportion of the intermediate product to the final product - amine.