Reactive scattering of low-energy ions from surfaces gives scattered product ions in which new bonds are formed with the adsorbate with a sensitivity to adsorbate geometry. Reactions of Cr+ and C5H5N+ , as well as chemical sputtering induced by Xe.+, are used to distinguished two well-characterized monolayer systems, namely 1,4-benzenedimethanethiol (BDMT) adsorbed on Au(111) and Ag(lll) thin films. While the reaction of Cr.+ with the Au monolayer produces an ion assigned as CrC7H5S+, this product is completely absent upon reaction with the Ag monolayer. Pyridine (C5H5N.+) projectiles abstract C-1-C-8 hydrocarbon groups in 50 eV collisions with the Au monolayer, while the Ag monolayer shows only C-1-C-4 abstraction with a significantly different intensity pattern. Chemical sputtering (Xe.+) mass spectra of the two surfaces an substantially different; complete fragmentation of the Ae adsorbate occurs, leading to C-2-C-4 ion ejection, while more of the molecular features are preserved in the spectrum recorded for the Au monolayer. The experimental facts are interpreted in terms of the geometry of the BDMT molecule that is monocoordinated on Au and dicoordinated on Ag. The experiments are extended to the 1,2- and 1,3-ring isomers of 1, 4-BDMT, which have not been characterized by other forms of surface spectroscopy. Ion/surface reactive collisions are useful for characterizing chemisorbates, including their geometrical orientation.