Adlayers of coadsorbed hydrogen (deuterium) atoms and methyliodide, H/CH3I, D/CH3I, H/CD3I, D/CD3I, were prepared on Ni(100) surfaces by adsorption of methyliodide on hydrogen (deuterium) saturated surfaces at 120 K. Upon directing thermal D (H) atoms at these adlayers as gaseous products hydrogen and methane molecules were observed which constitute of surface species only, e.g., H-2 and CH4 in the combination : D-->H/CH3I. The rates of formation of these products give evidence for reaction mechanisms in which adsorbed atoms, e.g., H, activated by collisions with incoming atoms, e.g., D, react with surface species, either H or CH3I to form H-2 and CH4. The reaction cross sections and isotope effects in H-->D/CD(H)(2)I or D-->H/CH(D)(3)I reactions support a collision:model in which:hot atoms act as mediators in the reaction pathways.