Knowledge of hydroxide ion's spectral signature is an essential ingredient for finding it at the water surface. In solutions, the intense hydrogen-bonded resonance of water often hinders unambiguous characterization of the OH- stretching frequency and linewidth. These parameters are likely to be modified at the hydrophobic water-vapor interface. This communication reports the first experimental observation of hydroxide ion that is free from interaction with bulk water. OH- is isolated in a hydrophobic environment and exhibits a sharp resonance at 3634 cm(-1), as confirmed by isotopic substitution. The feature is red shifted from that of dangling OH bonds by 29 cm(-1). (C) 2013 Elsevier B.V. All rights reserved.