The effects of Brij 35 micelles, CTABr micelles, and mixed Brij 35-CTABr micelles on the acid-base behavior of phenyl salicylate (PST) have been studied in aqueous solution containing 2% v/v acetonitrile, The apparent pK(b) (pK(b)(app)) of PST is decreased by 1.5 pK units with the increase in [Brij 35] from 0 to 0.02 M which is attributed to micellar medium effect. The values of pK(b)(app) remain almost independent of [CTABr] within its range 0.01-0.03 M. The increase in [CTABr] from 0 to 0.03 M in aqueous solution containing 0.02 M Brij 35 has not resulted in a change in pK(b)(app). This shows that the characeristic structural features of nonionic Brij 35 micelles remain essentially unchanged on addition of CTABr under the present experimental conditions.