The optical beam deflection technique (mirage effect) shows unambiguously that the ionic compensation of positive charges created in polyphenylene films on an electrode is affected only by the anion during the whole oxidation process. Use of a new technique of convolution by a frequency transfer function featuring diffusion in the electrolyte allows quantitative determination of the amount of exchanged species during cyclic voltammetry. The theoretical relationship between the mirage deflection and electrical current is derived for a symmetrical binary electrolyte exchanging only one ion with the interface, and when both diffusion and migration feature mass transfer. Measurement of the diffusion coefficient is possible during any electrochemical experiment by scanning the diffusion layer at various locations. Correction of the propagation delay between the electrode and the laser probe is possible with this convolutive approach. This allows the use of in situ optical beam deflection as a quantitative technique for studying dynamic mass exchange in complicated systems.