The chemical reactivity of glass surfaces is often studied with elemental analysis techniques, and although such characterization methods provide insights on compositional changes from exposure to specific chemical conditions, molecule-specific chemical reactions are not determined unambiguously. This study demonstrates the use of reflection-absorption infrared spectroscopy (RAIRS) to detect molecular species on alkali-free boroaluminosilicate and alkali aluminosilicate glasses, using acetic acid vapor as a model reactant to probe reaction sites at the surface with or without pretreatment by aqueous solutions of varied pH. With the assistance of the theoretical calculation of spectral changes based on refractive indices of bulk materials, it was possible to identify the molecular species being removed and produced at the glass surface. It was found that acetic acid vapor reacts with the modifier ions present at the glass surface to form monodentate acetate groups. Such reactions caused the desorption of 2-2.5 monolayers of strongly bonded water that remained on the glass surface, even after exposure to vacuum for several hours. The overall reactivity of the glass surface with acetic acid was found to be relatively insensitive to the pH of the pretreatment solution, although the glass surface composition and network vibrations were impacted and varied with the pH of the solution.