A bronze coin datable of the post-Roman empire and discovered in Morocco was studied. The in-depth structure and the composition of both patina and substrate alloy were determined by SEM observations, EDS (elemental dispersion analysis using X-ray) cartographies and Raman spectroscopy on a cross-section. The composition of the patina and that of the substrate alloy were found to be highly heterogeneous. Some patina particles were also scraped off from coin surface. Their electrochemical properties were characterized by cyclic voltammetry and electrochemical impedance spectroscopy with a cavity microelectrode. Cyclic voltammetry revealed the presence of three cathodic current peaks. In situ Raman microspectroscopy on the microelectrode allowed the attribution of two of them to Cu(0)|Cu(I) and Cu(I)|Cu(II). The third one was ascribed to the presence of lead oxide. Moreover, it was observed that tin dioxide initially present in the patina gradually disappeared independently of the potential cycling. This phenomenon was explained by the dissolution of this species through a chemical process. The impedance spectra collected with the cavity microelectrode were simulated suitably with an oxidation–reduction reaction coupled with a hemispherical diffusion model of dissolved oxygen.