An electrochemical Quartz Crystal Microbalance (EQCM) was used to characterize a mixed metal oxide electrocatalyst, Ir0.15Sn0.85O2, for the Oxygen Evolution Reaction (OER) in acidic medium in the potential range of the pseudo-capacitance reaction, from 0.4 to 1.2 V versus Reversible Hydrogen Electrode (RHE). The EQCM gold tip was characterized in H2SO4 0.05 M and HClO4 0.1 M. Subsequently, Ir0.15Sn0.85O2 powder, synthesized by a sol-gel route, was supported on the tip gold surface for investigations in the same media. The simultaneous measurements of mass variation and current density as functions of potential led to the identification of the chemical species involved in the mass transfer between the oxide and the acidic solution during the pseudo-capacitance reaction.