Quartz-crystal oscillators were operated in conjunction with film thickness monitors which convert the shift of the period of oscillation into indicated film thickness t. Films exceeding 125 kÅ of Cu (>415 kÅ of Al equivalent) were deposited on individual crystals. The deposit on glass slides simultaneously exposed to the deposition process was weighed. Up to the largest masses investigated, the indicated thickness of the deposit was linearly proportional to the deposit thickness on the slides. This result is unexpected since differences in the elastic constants of film material and quartz and/or changes of the oscillatory behavior could lead to deviations. Furthermore, the effective oscillating area of various crystals was only slightly larger than that of the deposit. The relationship between frequency shift and incremental deposit thickness was found to agree with theoretical predictions.