This paper reports detailed molecular beam measurements of the dissociative chemisorption probability for methane on a Pt(111) surface. We find large increases in the dissociative chemisorption probability S0 with increases in Ei cos2 θi (the normal component of translational energy), Ev (the vibrational energy of the incident methane), and Ts (surface thermal energy). The comparable activation of the reaction by addition of any of these three forms of energy cannot be accounted for by any single model for C-H bond activation proposed to date. A large kinetic isotope effect is also observed, with S0 decreasing significantly for CD4 relative to CH4. © 1989 American Institute of Physics.