The effect of fluorine substitution on the geometrical orientations and the reaction pathways of CH3CH2(a) and CF3CH2(a) on Ag(111) was investigated using temperature programmed reaction spectroscopy (TPRS) and reflection-absorption infrared spectroscopy (RAIRS). The CH3CH2(a) species couple to form C4H10(g) while CF3CH2(a) undergoes beta-fluorine elimination to give CF2=CH2(g). On the basis of the RAIRS data, the molecular axis of CH3CH2(a) is oriented nearly perpendicular to the surface while that of CF3CH2(a) lies much nearer the surface due to the interaction between a fluorine atom and the Ag(111) surface. We propose that anchoring of CF3CH2(a) by both Ag-C and Ag...F interactions increases the activation energy of CF3CH2(a) migration compared to CH3CH2(a) migration and, thus, suppresses migratory coupling and facilitates beta-fluorine elimination.