The chemistry of methyl bromide on Ru(001) has been studied utilizing work function change (Delta phi) measurements and temperature-programmed desorption (TPD) in the crystal temperature range of 82-1350 K. Employing a Delta phi-TPD mode, chemical changes in the adsorbed state could be detected at temperatures below the onset for desorption. A decrease in work function of 2.15 +/- 0.02 V has been measured at the completion of a monolayer coverage, which has been determined to consist of (3.6 +/- 0.3) x 10(14) molecules/cm(2), equivalent to CH3Br/Ru = 0.22 +/- 0.02. The onset for C-Br bond cleavage near 125 K was observed. 50% of the initial 1 monolayer methyl bromide molecules decompose to adsorbed methyl and bromine. A low-temperature increase in work function was found to precede dissociation or desorption as coverage increases. This change in work function is discussed in terms of several possible mechanisms, including multiple sites population, molecular rearrangement, and tilt angle, that change with coverage and surface temperature. A thermally activated flipping mechanism in which a fraction of the adsorbate molecules rearrange to adsorb with the methyl group facing the surface is found to be most consistent with the observed results. Sequential dehydrogenation of the methyl fragments, competing with minor methane production at higher coverages, was directly observed by employing the differential work function measurements. The corresponding surface temperature window for each of these decomposition steps has been determined, and the detailed reaction mechanism is discussed. Bromine atoms on Ru(001) were found to decrease the work function by 320 mV at a coverage Br/Ru = 0.3, indicating a complex charge redistribution upon adsorption. Deuterium preadsorption, which significantly passivates the surface, has been employed to better understand the various reactivity steps of the hydrocarbon fragments. Finally, work function measurements indicate the presence of strong interactions of the methyl bromide molecules with the metal surface up to the third layer. Alternating contributions to the work function of the first three layers are observed. This is understood in terms of an opposite adsorption geometry in which bromine faces the surface in the first layer, methyl in the second, and bromine again in the third. Upon heating, the third and fourth layers rearrange in a bilayer-like structure before the completion of the fourth layer, leading to higher stability of the combined two layers compared with the third alone. This structure is rather similar to that of methyl bromide in its molecular crystal.