Industrial steam reformers use temperatures greater than 1000 K to activate methane dissociation. At these temperatures, typical translational energies (E-trans) 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, S-0, for vibrationally excited methane in v = 1 of the v(3) C-H stretching vibration (E-vib = 36 kJ/mol) at E-trans ranging from 2 to 48 kJ/mol. On a 1000 K Ir(1 1 1) surface, v(3) excitation enhanced S-0 at all E-trans studied, and two distinct reaction channels appeared. When E-trans > 15 kJ/mol, S-0 increased with E-trans, as expected for direct dissociative chemisorption. When E-trans < 10 kJ/mol, S-0 decreased as E-trans increased. The low-E-trans 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 priorto 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. (C) 2014 Elsevier B.V. All rights reserved.