The kinetics of the reduction of toluidine blue by ascorbic acid was studied both in the absence and presence of hydrochloric acid as a function of cetyltrimethylammonium bromide concentration. The pseudo-first order rate constant was calculated from the decrease in the absorbance of toluidine blue. The results are analyzed according to pseudophase model of micelles. The reaction is catalyzed by cetyltrimethylammonium bromide micelles in the absence of hydrochloric acid but retarded in the presence of hydrochloric acid. This opposing effect of cetyltrimethylammonium bromide micelles may be explained by the changing species of reactants in acidic pH. To find a correlation between micellar effect on the rate constants and incorporation of the reactants to the micelles, separate spectrophotometric experiments were performed and binding constants for the association of reactants with micelles were determined using the Benesi-Hildebrant equation. (C) 2004 Elsevier Ltd. All rights reserved.