Industrial steam reformers use temperatures greater than 1000K to activate methane dissociation. At these temperatures, typical translational energies (Etrans) are much lower than the energy threshold for reaction, but the majority of methane molecules are vibrationally excited. A combination of molecular beam techniques and state-resolved infrared laser excitation allowed us to quantify reaction probability, S0, for vibrationally excited methane in v=1 of the ν3 C–H stretching vibration (Evib=36kJ/mol) at Etrans ranging from 2 to 48kJ/mol. On a 1000K Ir(111) surface, ν3 excitation enhanced S0 at all Etrans studied, and two distinct reaction channels appeared. When Etrans>15kJ/mol, S0 increased with Etrans, as expected for direct dissociative chemisorption. When Etrans<10kJ/mol, S0 decreased as Etrans increased. The low-Etrans results are consistent with a precursor-mediated mechanism in which vibrationally excited molecules first trap on the surface, sample different adsorption geometries and surface sites, and then react prior to vibrational quenching. The direct and precursor-mediated channels have nearly identical vibrational efficacies of 0.43 and 0.42, respectively, for promoting dissociative chemisorption. Our observations point to the potentially important role that vibrationally hot precursor molecules may play under thermal reaction processing conditions.