The binding of Al3+ to SDS micelles is best described by means of Guoy-Chapman-Stern isotherms. A method is described to estimate parameters for a Guoy-Chapman-Stern competitive adsorption isotherm for the system Na+/Al3+/SDS from data of the composition of micellar flocculates obtained in conditions of optimum flocculation. The dissociation coefficient KAl3+ is found to be equal to 17.67 if the bound fraction is defined as the ratio of bound Al3+ to SDS ions in the micelle. Experiments performed in the presence of a range of NaCl concentrations between 0 and 0.5 M show that the absence of flocculates in micellar solutions of lauryl sulfate in the presence of high [Al3+] cannot be explained by a drop in the chemical activity of Al3+ at high ionic strengths, although the chemical activity of Al3+ strongly influences the binding of Al3+ to SDS micelles. One effect of increasing [Na+] is to increase the critical micelle concentration of the surfactant in solution, from a value close to that of AI(DS)(3) to another close to that of SDS. This reduces the proportion of surfactant in micellar form and susceptible therefore of being flocculated.