The quality factor and frequency of vibration of a quartz crystal oscillator decreases slightly when a film adsorbs on its surface. Simultaneous measurements of the shifts in quality factor and frequency allow one to quantitatively deduce the degree to which the film ‘‘slips’’ due to the oscillatory motion of the underlying substrate, and also the force which would be required to slide the film along the surface at constant speed. Experiments have been carried out for krypton monolayers undergoing solidification on a smooth gold surface and on an atomically rough silver surface. The force per unit area required to slide a liquid monolayer is roughly the same for each surface. Less force is required to slide a solid monolayer along the smooth surface, while the opposite is true for the rough surface.