A true Arrhenius activation energy for the dissociative adsorption of H 2 on Cu(110) was measured using a Boltzmann distribution of H 2 gas at the surface temperature and found to be 14.3 ± 1.4 kcal/mol with a pre-exponential factor of 10 0.03±0.16 per H 2 collision with the surface. The temperature of the H 2 gas impinging on the heated Cu(110) surface was brought to the surface temperature ( ∼ 623 K) from the cold wall temperature ( ∼ 300 K) by increasing the total pressure in the reaction vessel with inert N 2. This increases the translational and internal energy of H 2 by collisional energy transfer near the surface. The rate of dissociative H 2 adsorption was found to increase strongly with the addition of N 2 up to ∼ 2 Torr, but to increase slowly above that, consistent with a “direct” mechanism for dissociative adsorption where the H 2 translational energy is most effective in scaling the activation barrier. Gas-phase collisional energy transfer was computer-simulated using known energy transfer rates to determine the average translational and internal energies of H 2 impinging on the copper surface as a function of N 2 pressure. The translational, rotational, and vibrational temperatures approach the surface temperature at widely different N 2 pressures, allowing assessment of the relative effectiveness of these degrees of freedom in assisting H 2 adsorption. Comparison of the activation energy with the desorption energy of adsorbed hydrogen indicates that the adsorption of hydrogen is nearly thermoneutral. Similar results are also reported here for D 2 adsorption, where no significant differences between D 2 and H 2 could be seen.