Frequency measurements in water for bulk acoustic wave (thickness shear mode) sensors with gold, aluminium, and silanized aluminum electrodes have been obtained by the oscillator method. Static and time-course responses for particular electrode-to-water interface conditions correlate with changes in hydrophobicity. The water-instigated alteration of the structure of silane films on aluminum electrodes has been studied by X-ray photoelectron spectroscopy, secondary ion mass spectroscopy, gas adsorption experiments, and determination of advancing contact angle with respect to water. The sensor responses are discussed in terms of perturbation of interfacial viscosity and the significance of this parameter in mediation of transfer of acoustic energy into a liquid.