The extreme surface sensitivity of metastable impact electron spectroscopy is utilized to probe the outermost electronic structure of two similar self-assembled monolayers (SAMs), 2-and 4-chlorobenzylmercaptan (CBM), adsorbed on gold films. All observed spectral features arising from the Cl substituent in 4-CBM are more distinct than those from Cl in 2-CBM, which confirms previous reactive ion scattering studies which showed that the SAMs are aligned upright on the surface, with the Cl in 4-CBM extending outwardly from the surface.