The adsorption of tetraphenylborate (TPhB(-)) and tetraphenylphosphonium (TPhP(+)) ions in self-assembled monolayers of phosphatidylcholine (PC) and phosphatidylserine (PS) deposited on a mercury electrode was estimated by measuring the time dependence of the charge which flows as a consequence of potential steps causing ion translocation across the monolayer. In the time scale of 100 ms, the translocation of TPhP(+) ions is controlled by diffusion from the bathing solution, while that of TPhB(-) ions is primarily controlled by the adsorption into the polar head region. Deviations from the Henry isotherm behavior were interpreted on the basis of an adsorption isotherm which accounts both for discreteness-of-charge effects and for the presence of two regions of different dielectric constant epsilon, i.e., the hydrocarbon tail region (epsilon approximate to 2) and the polar head region (epsilon(2) = 8-30). This isotherm also interprets a number of data for ionic adsorption in bilayers available in the literature using reasonable values for the parameters of lipid layers. The pH dependence of the adsorption coefficient of TPhP(+) and TPhB(-) ions in PS monolayers points to changes in the conformation and in the acidic properties of the polar heads of this lipid induced by ionic adsorption.