The kinetic and solvent isotope effects during the maleic acid heterogeneous catalytic hydrogenation and deuteration in the
light and heavy water have been studied. Also the effect of the γ and neutron irradiation on the Ni−ZnO catalysts (with various
ratios of components) on the reaction kinetics and mechanism has been measured, as well as the effect of pH on the adsorption
behaviour of maleic acid and the temperature depencence of the reaction rate. Existence of different adsorption centers for
hydrogen and maleic acid could be deduced from these experiments. A reaction mechanism based on the two-dimensional diffusion
of components in the surface is proposed.