A method for determining the equilibrium adsorption of an ionic surfactant on oppositely charged solid particles is proposed. Data from zeta-potential measurements are processed to obtain the adsorption constants of surfactant ions and counterions. The method can be applied to particles of arbitrary size and shape, but only if their characteristic dimension is much larger than the thickness of the electric double layer. The adsorption is quantified solely by zeta-potential experiments; additional measurements of concentration depletion and specific particle-surface area are not needed. The approach is suitable for concentrations below the threshold of hemimicelle formation. We carried out experiments on adsorption of dodecyl-trimethyl-ammonium-bromide on glass particles in the presence of 1 mM NaBr. The particles were obtained by crushing a glass plate to powder and subsequent screening to remove particles larger than 20 mum. Control experiments with spherical particles from the same material showed that the method is insensitive to particle shape. Since the surfactant (DTA(+)) and Na+ bear the same electric charge, they compete with each other in the adsorption at the negatively charged glass. This is accounted for in the theoretical model. The binding of Na+ counterions is found to play an essential role in the overall adsorption process.