The influence of a monolayer of a lipid, distearoyl phosphatidylcholine, formed at a water I dichloroethane interface in a special Langmuir trough, which allows simultaneous control of the state of the monolayer and the potential across the interface, on the transfer of ions across this interface has been studied using two probe ions: the cationic drug propranolol and anionic picrate, From measurement of the isotherms of the lipid monolayer and of the cyclic voltammograms of propranolol and picrate at controlled surface pressure it was found that picrate stabilises the monolayer and that its transfer across the interface requires more energy in the presence than in the absence of lipid. Propranolol. however, has a destabilising influence on the monolayer and accelerates lipid desorption. The kinetics of transfer of both ions was too fast to be followed by cyclic voltammetry.