The interaction of methane and methane-d4 on nickel has been re-examined. The data strengthen the previous concept of a dissociative adsorption of methane on the catalyst. The kinetic data indicate first-order disappearance of methane-d4 with an activation energy of 20.9 kcal. between 100° and 255 °C. Poisoning by carbonaceous residues occurs at all temperatures. First-order kinetics are indicated for the formation of methane-d3 and methane-d2 on the catalyst surface. Reaction of hydrogen with surface residues, after an exchange reaction, indicate that CX, CX2, and CX3 fragments are present on the surface where X is H or D. Higher temperatures favor an equilibrium between these fragments on the surface, equilibrium being displaced towards CX3 as temperature increases. This equilibrium will be dependent on the heats of adsorption of the fragments and of hydrogen on the surface and therefore involves also the metal used as catalyst. The data suggest a basic approach to the mechanism of the Fischer–Tropsch synthesis on metal catalysts.