The chemistry of methyl bromide on Cu/Ru(001) has been studied utilizing work function change (Delta phi) and temperature-programmed desorption (TPD) measurements. The remarkable modification in the methyl fragments dehydrogenation at the completion of a single copper layer and the significant difference in reactivity of the Cu(2 ML)/Ru(001) or Cu(lll) surfaces are the focus of this study. A decrease in work function at the completion of 1 ML CH3Br of 2.15 +/- 0.02 eV and 1.33 +/- 0.05 eV was measured, respectively, for Ru(001) and Cu(2 ML)/Ru(001) held at 82 K. Methyl bromide does not dissociate upon adsorption on clean or the copper-covered surfaces, and it is bound with the bromine down. Copper modifies the reactivity of the Ru substrate, gradually decreasing the dissociated fraction of CH3Br from 0.55 of the initial one monolayer on clean Ru(001) to 0.06 on Cu(2 ML)/Ru(001), probably because of defects in the copper layer. The methyl fragment dehydrogenation rate slows as the copper coverage increases. At a narrow copper coverage range between 0.8 and 0.95 ML, adsorbed hydrogen and methyl fragments coexist on the surface in the temperature range 230-280 K. Sequential. decomposition channels of the parent molecules and the methyl fragment lead to a unique enhancement of methane production rate, this on the account of further hydrocarbon dehydrogenation, as reflected in both Delta p and Delta phi TPD measurements. Methane is formed on top of copper terraces as a result of "spill-over" of both methyl and hydrogen atoms, similar to the chemistry over Cu(lll) and Cu(110) single-crystal surfaces. The dipole moment of adsorbed methyl is reported here for the first time on metal surfaces, being 0.48 D on top of Cu(2 ML)/Ru(001).