Gas-surface scattering experiments are used to probe the dynamics of energy transfer and thermalization in collisions of 80 kJ/mol Ar with self-assembled monolayers composed of SH-(CH2)(11)-CH3 and SH-(CH2)(11)-OH alkanethiols on gold. We find that the extent of thermalization and the fractional energy transferred to the surface during the collision are significantly greater for the CH3-terminated surface than for the OH-terminated surface. Since the two monolayers are similar in structure, packing density, and mass, the differences in scattering dynamics are likely due to a combination of factors that may include differences in the available energy modes between the two terminal groups and the hydrogen-bonding nature of the OH-terminated alkanethiol self-assembled monolayer.