The Potential Energy Surfaces of the dehydrogenation reaction of NH3 and CH4 molecules by the first-row transition metal cations Mn+ (S-7, S-5) were investigated employing the Density Functional (B3LYP) and the CCSD(T) levels of theory. A close description of the reaction paths leading to three dissociation products was given, including several minima and key transition states. The reactions proceed to give dehydrogenation products by oxidative addition of the metal cation into one of the H-X bonds (X = N,C) and formation of the H-M+-XHn-1, hydrido intermediates, which in these cases are also confirmed to represent stable minima along the quintet surface. Because the spin state of the reactants is different from that of intermediates and products an intersystem crossing is proposed to occur. The binding energies of reaction products were calculated and compared with available experimental data to calibrate the quality of our approach.