The dissociative chemisorption of CH4 and N2 on Ni(poly) has been studied using molecular beam techniques. Chemisorption probabilities have been measured as a function of translational kinetic energy. The initial sticking probability was found to depend strongly on the incident energy Ek, scaling En =Ek cos 2θi. No dissociative chemisorption was observed for CH4 below En =44.0 kJ/mol and for N2 below En =46.1 kJ/mol. Above these threshold values, the initial sticking probability increases linearly from 0 to 0.54 as En increases from 44.0 to 47.9 kJ/mol for CH4 and from 0 to 0.03 as En increases from 46.1 to 83.3 kJ/mol for N2 . These results indicate that these systems proceed by a rather direct chemisorption process, rather than by a classical precursor mechnanism. The variation of the initial sticking probability with ‘‘normal’’ kinetic energy s0(En ) is analyzed to obtain the barrier height or apparent activation energy as 46.6 and 50.2 kJ/mol for CH4 /Ni(poly) and N2 /Ni(poly) systems, respectively. Finally, comparison with previous results and pressure gap effects in heterogeneous catalysis are discussed.