A simple method for studying the reactivity of hydrogen-terminated silicon surfaces via transmission FTIR spectroscopy is described. Abraded (100) single crystals of silicon, after etching in ammonium fluoride, show features in the IR spectrum that are assigned to Si–H stretching vibrations. Confirmation of the utility of this approach is demonstrated in the reactions of the Si–H surfaces with water and methanol. Native-oxide growth in the former case is evidenced by the appearance of two bands in the Si–O stretch region accompanied by a shift to higher frequency of the longitudinal polar Si–O vibrational mode with increased exposure, consistent with an island-growth model. Similarly, the reaction with methanol generates two bands in the Si–O stretch region. However, the frequency and behaviour of these bands on further exposure to methanol are consistent with a chemisorbed methoxy species with little oxidation taking place, contrary to previous reports.