Infrared spectra in the region 4000–1300 cm−1 have been obtained of acetic acid, acetyl chloride, acetic anhydride, propionic acid, and propionyl chloride adsorbed on discs of pressed silica powder. In all cases the spectra showed the presence of both the simple hydrogen-bonded species[Formula: see text]where X = OH, Cl, or O.CO.R and Si* represents a surface silicon atom, and also chemisorbed surface ester groups R.CO.O—Si*. The latter were characterized by a νC=O frequency in the 1760–1740 cm−1 region, whereas in the case of hydrogen-bonded physical adsorption, the νC=O frequency was about 20–30 cm−1 lower than that of the same molecule in the vapor phase. At high temperatures (150–200 °C) the water or hydrogen chloride eliminated in the surface esterification reaction was desorbed, and high conversions of surface OH to surface ester groups were observed. Under these conditions the carboxylic acid was formed from both the chloride and the anhydride; the latter was also produced at room temperature from the acid chloride. A scheme is suggested for the equilibration of these compounds on hydroxylated silica surfaces. The chemisorbed ester groups were susceptible to hydrolysis by water vapor, thus regenerating the original silanol OH groups.