Abstract The relationship between voltammetric charge and BET surface area was investigated for a series of RuO 2/TiO 2-coated titanium electrodes spanning the entire range of composition. Despite the fact that both parameters varied in a rather complex manner with oxide composition at constant loading, a simple linear relationship was established between the charge per unit area and the RuO 2 content of the surface layer. This permits extension of the voltammetric technique for measuring surface area, described originally for pure RuO 2, for use with the mixed oxide system. The results confirm that RuO 2 is the electroactive component in this binary system and indicate that surface enrichment does not occur with RuO 2/TiO 2 mixtures. Comparison with results obtained by other authors suggests, however, that complications may arise in the case of very thin films which appear to be modified by interaction with the support. It is proposed to investigate the latter effect in more detail shortly.