Rh-catalysed Fischer-Tropsch synthesis at high pressures exhibits reversible electrochemical promotion by potassium. At 473 K and 14 bar, 43 products were detected, comprising alkanes, alkenes and primary alcohols. Potassium increased selectivity towards alkenes and primary alcohols as a result of actual increases in the rates of formation of these products - an apparently unprecedented observation. Alkane formation was progressively suppressed by added alkali. These findings are discussed in terms of K-induced acceleration and retardation of possible rate limiting steps for alkene and alkane formation, respectively. Plausible mechanisms are discussed.