The catalytic performance of rhodium thin films in contact with the solid electrolyte Na beta " -alumina can be greatly enhanced by electrochemical promotion. In the reduction of NO by propene, increases in overall activity by a factor of 2.4 can be achieved accompanied by a gain in nitrogen selectivity from 45% to 82%. These improvements are most pronounced under reducing conditions and are unaffected by deliberate addition of CO2. XPS data show that the effect is due to reversible transport of Na between the solid electrolyte and the metal film catalyst whose potential, measured with respect to a reference electrode, determines the sodium coverage. Catalytic promotion is due to the Na-induced dissociation of NO, the key reaction-initiating step. Under reaction conditions, the sodium is present as carbonate, some of which is in the form of 3D crystallites. Comparison with corresponding data obtained with conventional dispersed Rh/gamma -alumina catalysts shows that Na promotion of the latter is due to the effect of alkali on the surface chemistry of the metal component; effects on the support must be negligible.