The present investigation was undertaken to examine the effects of added sodium chloride on solubilized systems of sodium lauryl sulfate (SLS) micellar solutions solubilizing alkylparabens by gel filtration. The results showed that the addition of sodium chloride decreased the distribution coefficients of alkylparabens between the SLS micellar and aqueous phases. The factors causing the decrease of distribution coefficients were investigated. Activity coefficients of SLS and alkylparabens in mixed micelles were estimated on the basis of ideal solution theory. The activity coefficients suggested that the addition of sodium chloride hardly changed the SLS-alkylparaben interaction in the mixed micelles. The decrease in the surface area per SLS molecule in the micelles on addition of sodium chloride seems to be an important factor.