A theoretical expression is given for the electrostatic potential ((\Delta V)) of a monolayer consisting of long-chain surfactant molecules, water molecules, and adsorbed 3-phenylindole (3PI) molecules of a slightly soluble compound. The theory incorporates the effect of bending of the surfactant chains, due to which a portion of the chain is immersed in water and preferentially orients nearby water molecules by molecular interaction. Since water molecules have a large electric dipole moment, the bending thus affects the value of (\Delta V). The length of the immersed portion depends on surface pressure ( (\Pi) ), so that the change in (\Delta V) also depends on (\Pi). Experimental data obtained in the liquid expanded state with long-chain lipids (phosphatidylcholine/cholesterol) but without 3PI molecules indicate a good fit of the theoretical expression of (\Delta V) as a function of (\Pi). However, a unique model has to be introduced in order to account for the influence on (\Delta V) of 3PI molecules added at various bulk concentrations to the two-component system over a wide range of (\Pi). Namely, lipid polar heads create a strong trapping potential for 3PI molecules at the air-water interface as well as beneath the polar heads. 3PI molecules also have a large electric dipole moment and thus affect (\Delta V, \odot 1993) Academic Press, Inc.