The results of the present investigation demonstrate that the interfacial regions of cetyltrimethyl-ammonium bromide (CTAB)/isooctane/1-hexanol/water cationic reverse micelles are densely populated by bromide counterions (3.7-2.5 M) and cosurfactant (8.1-6.4 M) across W-0 (the mole ratio of water to surfactant) = 12-44 and the interfacial concentrations remain grossly unchanged in the presence of a catalytic amount of interfacially solubilized lipase at both pH 6.0 and 7.0 (pH refers to the pH of the aqueous buffer solutions used in preparing the reverse micellar solutions). Both the second-order rate constants k(2) and the equilibrium yields for the lipase catalyzed hydrolysis of p-nitrophenylcaproate were also found to be essentially unchanged across the entire range of W-0. Our results throw new insight into the origin of such constant second-order rate constants and equilibrium yields for lipase-catalyzed interfacial hydrolysis reactions in cationic reverse micelles across W-0 = 12-44. To our knowledge, the work delineated herein provides the first experimental evidence consistent with grossly unchanged interfacial compositions as the origin for the unchanged activity profiles and the equilibrium yields for lipase-catalyzed reactions in cationic reverse micelles across a wide range of W-0.