In the partial hydrogenation of acetylene, potassium, pumped to a thin film Pt catalyst from the electro-active support K beta"-alumina, very strongly promotes ethylene formation. In the best case, K-promotion increases ethylene selectivity from <20% (for the un-promoted catalyst) to 90%. XPS and photoelectron microscopy show that under these conditions submonolayer amounts of K are uniformly distributed on the Pt surface and that alkali transport, like the catalytic behavior, is fully reversible as a function of catalyst potential. Auxiliary experiments strongly suggest that ethylene, or a surface species derived from it, is a key intermediate in the total hydrogenation of acetylene to ethane. Promotion of selective hydrogenation by potassium may be rationalized in terms of K-induced weakening and strengthening respectively of ethylene and hydrogen adsorption. Comparison with earlier results indicates that the associated electronic effects are relatively insensitive to the chemical identity of the alkali.