With the use of a multisector ion/surface scattering mass spectrometer, the benzoyl cation, C6H5CO+, is mass- and energy-selected and then made to collide at hyperthermal energies (i.e., < 100 eV) with an HO-terminated self-assembled monolayer (HO-SAM) surface. Fourier transform infrared external reflectance spectroscopy (FTIR-ERS) indicates that this ion/surface collision results in C-O bond formation at the surface, producing the terminal benzoate. The covalent modification of the hydroxyl surface is further substantiated by subsequent ion/surface scattering experiments, in which 70-eV CF3+ ions are used as projectiles. Chemically sputtered ions resulting from the collision of the CF3+ I ion with the ion-modified surface include the reagent ion, C6H5CO+, and its fragments, C6H5- and C4H3-. The same chemically sputtered ions are observed when a C6H5CO2-terminated self-assembled monolayer surface is similarly analyzed using CF3+, Using collisions of CF3+ to analyze a series of mixed SAM surfaces (prepared from varying amounts of the HO-terminated disulfide and the C6H5CO2-terminated disulfide), calibration of the chemically sputtered products reveals that a 2-h modification with the benzoyl cation results in ca. 15% reaction yield (i.e., 15% of the surface groups are converted to products). The reaction efficiency (i.e., fraction of gas-phase cations converted to surface-bound products) is roughly estimated as 75%, the first ion/surface reaction efficiency to be reported. Analogous surface transformations are achieved using (CHCO+)-C-3 and C6H5CH2+.