2,6-Diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenoxide, ET(30), has been investigated in order to ascertain its suitability as a probe for both the effective interfacial dielectric constant (ε eff) and the electrostatic surface potential (Ï 0) of model lipid membranes in aqueous solution. This work establishes that the solvatochromic visible absorption band for ET(30) can be used to provide a good estimate of the ε eff for cationic micelles. It is also shown that the acidâ base dissociation of ET(30) can be utilized to obtain a quantitative measure of the Ï 0 in the case of cationic micelles. There are problems and uncertainties associated with the use of ET(30) in aqueous solutions of other types of charged self-assembled surfactant aggregates, and these are discussed.