Abstract The catalytic activity and selectivity of metals can be reversibly altered by supplying promoters electrochemically. This has been achieved by using a catalyst as the working electrode in a solid electrolyte electrochemical cell. In this letter we review the accepted explanation for the relationship between the catalyst work function ( eφ W) and the potential difference between the working and reference electrodes of the electrochemical cell ( V WR). We show that this explanation is not correct. A theoretical derivation for the relationship found between eφ W and V WR in some experiments is not presently available. For the moment, it must be considered only an experimental finding.