In micellar-enhanced ultrafiltration (MEUF), solubilization of organic solutes in micelles, combined with ultrafiltration of these micellar solutions, leads to the removal of dissolved, low molecular weight organic compounds from water. The permeate from MEUF contains the organic solute at concentrations equal to the unsolubilized solute concentration in the rejected (retentate) solution. Therefore, the equilibrium solubilization of the solute dictates the permeate purity or rejection of the solute by the membrane. The semiequilibrium dialysis method has been used to investigate the equilibrium solubilization of o-, m-, and p-cresol by aqueous solutions of 1-hexadecylpyridinium chloride, throughout a range of concentrations of the cresols and the surfactant. The apparent solubilization constant, K = X/sub c//(concentration of unsolubilized cresol), has been correlated with X/sub c/, the mole fraction of cresol in the micelle.