Calcareious deposits were investigated by electrochemical impedance spectroscopy for electrochemically accelerated scaling. Firstly, the oxygen reduction was studied in a solution of low ionic strength similar to the usual scaling water solutions. Its reaction mechanism can be described by an electron transfer limited by the mass transport in the solution bulk. Secondly, by measuring the impedance on a totally covered electrode at the end of scaling it was shown that dissolved oxygen reduction occurs, through the porous scale, in cavities under the blocks of calcite crystals. The resistance and capacitance of the scale layer was investigated by measuring the impedance at zero current. Impedance measurements give information on the compactness and the thickness of the deposit. Scanning electron microscopy revealed that the morphology of the scale deposit is dependent on mass transport in solution.