The interactions of methyl chloride and methyl bromide on Ru(001) have been studied and compared using work function measurements in a temperature-programmed desorption (Delta phi-TPD) mode and normal temperature-programmed desorption(Delta p-TPD). Both molecules adsorb molecularly at temperatures below 100 K with the halide facing down, forming direct bonding to the metal and reducing the work function by 1.9 and 2.1 V, for CH3Cl and CH3Br, respectively. Dipole moments of the isolated molecules and their polarizabilities were extracted by using electrostatic models for the work function as a function of coverage. While the crystal is heated, methyl chloride desorbs molecularly with no trace of dissociation. This is concluded from the similarity of the differential work function change (DWFC) and the Delta p-TPD spectra. In contrast, 50% of an initial one monolayer of methyl bromide dissociates to form adsorbed methyl and bromide. Significant differences between the DWFC and the normal desorption spectra are discussed in terms of the sensitivity of work function to rearrangement of the molecular species at low temperatures and to dissociation of the parent molecule and its methyl fragments at higher temperatures.