Both in semiequilibrium dialysis (SED) and micellar-enhanced ultrafiltration (MEUF) experiments, it has been shown that cationic hexadecylpyridinium chloride (CPC) micelles are very effective in binding oppositely charged arsenic ions in the retentate through electrostatic interactions. The resultant colloid is large enough not to pass through the membranes, producing a rejection nearly 100%. Because the equilibrium process is similar in the SED technique and the analogous MEUF experiments, the unbound arsenic concentration in the SED technique is practically the same as the permeate concentration of these species in MEUF experiments. In order to correlate the binding of arsenic to CPC in SED and MEUF, a nonlinear equilibrium model which combines thermodynamic relations, charge balance equations, and material balance equations with the Oosawa two-phase polyelectrolyte theory has been developed. The equilibrium model successfully accounts for the experimental data in both SED and MEUF in the absence and presence of monovalent and divalent co-ions.