Interfacing a Pb2+ conducting solid electrolyte with a thin-film Pt catalyst permits in situ electrochemical control of surface composition and catalytic performance of the resulting bimetallic catalyst. Transport of Pb between the solid electrolyte and the Pt film provides a controlled, reversible, and reproducible way of altering the surface composition of the Pt/Pb-alloy surface. Large increases in acetylene partial hydrogenation selectivity may thus be achieved from 20% with pure Pt to 85% with a similar to26% Pb-alloy surface. This is the first demonstration that the surface composition and catalytic performance of bimetallic-alloy catalysts can be electrochemically controlled under working conditions.