Pseudo-first-order rate constants (k(obs)) for alkaline hydrolysis of 4-nitrophthalimide show a monotonic decrease with increase in [C12E23](T) (total concentration of Brij 35) at constant [CH3CN] and [NaOH]. This micellar effect is explained in terms of a pseudophase micelle model. The rate of hydrolysis becomes too slow to monitor at [C12E23](T) greater than or equal to 0.03 M in the absence of cetyltrimethylammonium bromide (CTABr) and at [C12E23](T) greater than or equal to 0.04 M in the presence of 0.006-0.02 M CTABr at 0.01 M NaOH. The plots of k(obs) versus [C12E23](T) show minima at 0.006 and 0.01 M CTABr, while such a minimum is not visible at 0.02 M CTABr.