Thanks to the piezoelectric behaviour of quartz and its adoption in the quartz crystal microbalance (QCM), a technique is available which allows the minutest changes in mass to be followed as a function of time. Frequency shifts as a function of time provide valuable information, for example on the mass or thickness of an electroplated or chemically deposited coating. Growth or deposition rates as well as current efficiencies of electrochemical processes are likewise accessible in this way. Time resolution is of the order of a few milliseconds, mass resolution of the order of nanograms. Using this technique one can, for example, follow the change in current efficiency of copper deposition from an acid bath due to inhibition resulting from the adsorption of organic molecules or the formation of an electrochemical double layer. One application of the quartz crystal microbalance reported here is the study of very thin electroplated films in the range 5 to 20 nm.