The electropolymerization of methacrylonitrile (MAN) and N-vinyl-2-pyrrolidone (NVP) in anhydrous acetonitrile has been followed in situ using an electrochemical quartz crystal microbalance (EQCM). These two monomers essentially differ in that MAN leads to both a grafted polymer and a polymer formed in solution,while NVP only delivers the grafted polymer. For both molecules, our results indicate that the frequency response of the EQCM anticipates the voltammetric response. In the case of MAN,the admittance of the EQCM shows a considerable increase which is synchronized with the voltammetric peak. Alternatively, in the case of NVP, the admittance of the EQCM remains almost constant during the course of the experiment. These results lead us to the proposal that: (i) the eigenfrequency variations of the EQCM may be related to mass changes on the surface of the quartz crystal in the case of the electropolymerization of NVP molecules over the whole potential range, while the same is true only over a few hundreds of mV before the peak in the case of MAN; (ii) poly-NVP films grow directly from the surface of the electrode, while at least two regimes are detectable in the case of MAN.