This paper describes an investigation of the charging processes of Rh2O3 electrodes in acidic medium using Electrochemical Quartz Crystal Microbalance. The Rh2O3 was prepared by the Pechini method. The microstructural characterization of the rhodium oxide was performed using Scanning Electron Microscopy and the structure was determined by X-ray diffraction. The Rh2O3 oxidizes at potentials higher than 0.8 V. A mass loss of 60 ng was observed during the anodic sweep. The same amount is gained during the cathodic sweep indicating that the process is reversible. From the mass versus charge plots a slope of 8.5 g mol(-1) is calculated. Considering a process that involves a two-electron transfer, the oxidation of Rh2O3 to RhO2 with the loss of a water molecule (18 g mol-1) is proposed. (c) 2005 Elsevier B.V. All rights reserved.