The solvatochromic acid-base indicator 2,6-diphenyl-4-(2,4-6-triphenyl-1-pyridinio)phenooxide, ET(30), is employed to determine the effective surface potentials of micelles (Ψ0expt) in relatively concentrated aqueous dodecyltrimethylammonium chloride (DTAC) systems (i.e. up to 40 wt% DTAC). Ψ0expt decreases with increasing DTAC concentration, This is attributed to a concomitant increase in the amount of dissociated chloride ions in solution when the DTAC micellar concentration is increased. The effective surface potentials are compared with the electrostatic surface potentials obtained in relatively dilute micellar DTAC/NaCl systems and with the electrostatic surface potentials calculated from the non-linearized Poisson-Boltzmann (P-B) equation and a cell model (Ψ0cell). The P-B cell model, which incorporates a Langmuir adsorption constant in order to categorize the association of Cl− to the surfactant headgroups, adequately describes Ψ0expt up to 7 wt% DTAC. Beyond 7 wt% Ψ0expt and Ψ0cell progressively diverge. The divergence is primarily considered to be a result of the fact that the P-B cell model neglects the ion-ion correlations and the finite size of the ions.