The thickness-shear-mode quartz resonator, known as an electrochemical quartz-crystal nanobalance (EQCN), is widely used as a mass-sensitive detector in microgravimetry electrochemical experiments. Using the acoustic-wave model, we show that in experiments where the load film is involved in some chemical reaction, the resonance-frequency shifts are due not only to thickness-related mass changes in the load film, but also to changes in the film density or acoustic properties. Such effects may take place during electrochemical experiments such as corrosion, ion intercalation, oxidation-reduction, phosphatation, or any other chemical modification of the film. In such eases, the resonance-frequency shifts are due to the change both of mass and of resonator-film acoustic coupling. The Sauerbrey relation does not take into account the effect of acoustic coupling; the use of this relation in electrochemical gravimetry may lead to erroneous conclusions. The change in resonance frequency due to changes in density or acoustic properties may, under some circumstances, provide information concerning the chemical nature of the load film.