In this work, the voltammetric charge and work function of Rh catalyst were studied to clarify the origin of electrochem. promotion. The measures were carried on at 375 Deg, either in a non-reactive oxygen/helium gas mixt. or under complete oxidn. of propylene. The voltammetric charge measured was strongly dependent of the scan rate and of the surface area of the catalytically active Rh/gas interface. On the base of this result, a model, analogous to that of emersed electrodes in liq. electrochem., was proposed, which postulates the presence of ions coming from the solid electrolyte on the gas exposed catalyst surface. The maximal voltammetric charge that can be stored at the gas exposed catalyst surface (outer charge), detd. by Trasatti's method, increased with the catalytically active Rh surface. The capacity of the Rh/gas interface was calcd., and values from 3 to 200 mF were obtained for different catalyst loadings. Work function measurements were used to study the Rh gas exposed surface under steady state conditions. In oxygen/helium atm. the one-to-one relation between work function and potential was confirmed over the whole potential domain studied. This relation was not valid anymore when stoichiometric oxygen to propylene mixt. was used. [on SciFinder (R)]