By using electrochemical and EHD impedances, CaCO3 accelerated deposition from carbonically pure waters on a rotating platinum disc electrode can be characterized through the kinetics of oxygen reduction on a partially blocked surface. An approximate model of the electrochemical impedance is proposed which explains the main features of the experimental data. The electrochemical impedance, measured during scale growth, allows two quantities, R(HF) and C-HF Obtained from its high frequency behaviour, to be determined. C-HF can be related to the scale coverage of the surface and R(HF) to the morphology of the scale deposit. In addition SEM observations illustrate the scale deposit growth process at the electrode surface.