The amplitude of vibration of a quartz crystal oscillator decreases slightly when a film adsorbs on its surface. Measurement of this decrease in amplitude allows one to quantitatively deduce the solid–liquid interfacial viscosity and the degree to which the film ‘‘slips’’ due to the oscillatory motion. Experiments have been carried out on various ‘‘model interfaces,’’ including xenon and nitrogen on gold, and water and cyclohexane on silver. Monolayers and bilayers of these films are observed to exhibit greater average slippage than thick films of the corresponding bulk material. The more massive monolayers are observed to exhibit shorter slip times.