The wettability and stability of silane-treated substrates are central in many processes of industrial and environmental interest, Here, we bring the capability of X-ray photoelectron spectroscopy (XPS) to bear on the wettability evaluation of treated glass. For a suite of silane-treated slides we investigate the relationship between wettability and surface composition as determined from XPS, The adhesion behavior and contact angle of mineral oil on brine-covered surfaces are used here as surface wettability indicators. The XPS spectra of silanes and silane-treated glass slides indicate that silicon species of the kind Si-CH exhibit a well-defined binding energy which differs from that of the Si-O species of the glass. The presence of the Si-CH species reveals glass surfaces that have been rendered hydrophobic by chemisorption of organic matter. The surface carbon content as determined from XPS correlates strongly with the equilibrium contact angle, contact angle hysteresis, adhesion behavior, and wettability state. This correlation can be used to rapidly determine the wettability of treated glass.