Sodium supplied to the surface of a ruthenium thin film catalyst by electro-pumping from a solid electrolyte (Na-beta " -alumina) strongly alters the activity and selectivity of the latter in Fischer-Tropsch synthesis. Thus the range of utility of electrochemical promotion has been broadened, this being the first application to a C-C bond forming reaction. The methanation rate is strongly suppressed resulting in a marked increase in selectivity towards C-2-C-4 hydrocarbons, accompanied by an increase in the alkene : alkane ratio. The results obtained with this model system, including the "C-2 anomaly", are in close agreement with those found for classically promoted conventional dispersed catalysts. Alkali substantially increases the probability of chain growth and CO dissociation is not rate controlling; mechanistic implications are discussed.