Comparative reactivities have been determined for the cleavages of p-nitrophenyl phosphotriester substrates 4-9, and 12, and 13 by iodosobenzoate (1) and iodosonaphthoate (11) in micellar cetyltrimethylammonium chloride at pH 8. The aggregate substrate reactivities can be manipulated by altering the phosphate substituents, with each PhO-to-MeO or -EtO change worth similar to 1 order of magnitude decrease in the rate constant. With 2 PhO/EtO mutations, the reactivity of 4 is 470 times greater toward 1 than that of 5, and the differential increases to 1080 with nucleophile 11. Phosphodiester substrates such as 10 are inert to micellar 1. Reactivity differences can be amplified by covalent attachment of the substrate moiety to cationic surfactants (12 and 13), where the PhO/EtO changes induce 600-700-fold reactivity diminutions with nucleophiles 1 and 11. The differential reactivities of the various substrates reflect both innate reactivity differences and differences in binding to the micellar pseudophase.