Sum-frequency vibrational spectroscopy was used to probe the adsorption of C1-C4 1-alcohol molecules at both vapor/silica and liquid/silica interfaces. The observed CH stretching spectra show that alcohol molecules adsorbed on silica with methyl groups pointing away from the interface. Reduction of the symmetric modes and enhancement of the antisymmetric modes of alcohol at liquid/silica interfaces are understood from a model of oppositely oriented molecules forming a bilayer at the interfaces. (c) 2005 Elsevier B.V. All rights reserved.