The applicability of electrochemical promotion (EP) has been extended to an investigation of the Pt-catalysed hydrogenation of acetylene to ethylene and ethane, the first example of the use of EP in catalytic selective hydrogenation. The system exhibits strong promotion by spillover Na supplied from a beta "-alumina solid electrolyte. In the hydrogen-rich regime, reversible, potential-dependent, increases in ethylene selectivity of an order of magnitude are achievable under the influence of similar to 0.02 monolayer Na, while maintaining high acetylene conversion. Thus, EP by Na induces Pt, normally a poor selective hydrogenation catalyst, to behave like Pd. Butenes, butadiene, and benzene are observed as minor products whose formation is also promoted by Na. A simple reaction model based on the destabilising effect of charge transfer from Na to adsorbed ethylene accounts for the principal findings.