We describe the dissociative attachment (DA) of methyl bromide to form chemisorbed CH3 and Br on a Si(100)-2 x 1 surface at 270 K. The patterns of DA were studied experimentally by ultra-high vacuum scanning tunneling microscopy (STM) and interpreted by ab initio theory. The parent molecules were found to dissociate thermally by breaking the C-Br bond, attaching the resulting fragments CH3 and Br at adjacent Si-atom sites. The observed DA resulted in three distinct attachment geometries: inter-row (IR, 88%), inter-dimer (ID, 11%), and on-dimer (OD, 1%). Ab initio computation agreed in predicting these three DA reaction pathways, with yields decreasing down the series, in accord with experiment. The three computed physisorption geometries, each of which correlated with a preferred outcome, IF., ID, or OD, exhibited similar heats of adsorption, the choice of pathway being governed by the energy barriers to DA chemisorption predicted to increase along the series: E-IR = 0.48 eV, E-ID = 0.57 eV, and E-OD = 0.63 eV.