Pseudo-first-order rate constants (k o b s ) for the alkaline hydrolysis of phthalimide (PTH), obtained at a constant concentration of cetyltrimethylammonium bromide (CTABr) and 35 o C, vary with the increase in the concentration of sodium phenylacetate and sodium sulfate (MX) according to the relationship: k o b s =(k 0 +θK[MX])/(1+K[MX]) where θ and K are empirical parameters. The value of θ/k h W (where k h W =k o b s at [CTABr] T =[MX]=0) is the measure of the extent of expulsion of ionized PTH (S - ) from CTABr micellar pseudophase to aqueous pseudophase by an inert anion (X - ) provided S - is fully micellized in the absence of X - . Thus, the values of θ/k h W for sodium phenylacetate (~1) and sodium sulfate (~0.12-0.19) are attributed to deeper micellar penetration of S - and phenylacetate ions compared to that of sulfate ions.