Comicelles of (Z)-dodecano-N-phenylhydroxamate ion (Ib) with cationic and sulfobetaine surfactants effectively accelerate the hydrolysis of p-nitrophenyl diphenyl phosphate(pNPDPP). The surfactants are cetyl trimethyl- and tributylammonium bromide (CTABr and CTBABr respectively), and (tetradecyldimethylammonio)propanesulfonate (SB3-14). Reaction is slower in comicelles with Brij-35, 23-dodecyl ether (C12E23), and much slower in comicelles with sodium dodecyl sulfate(SDS). In all conditions, the hydroxamic acid was fully deprotonated and pNPDPP was fully micellar-bound and, on the basis of NMR data (Langmuir 1997, 13, 6439), it appears that the rate effects are due to differences in the relative locations of pNPDPP and the hydroxamate residue in the interfacial region, which are related to micellar charge. Inhibition of reaction in SB3-14 on addition of ClO4-, and acceleration in SDS by weakly hydrophilic cations, are understandable in these terms, as are effects of n-dodecyldimethylphosphine oxide and n-decylmethyl sulfoxide on reactivity in SDS. The propiono-N-phenylhydroxamate ion (la) is an E-Z mixture in water, which complicates comparisons of rate constants of this reaction with pNPDPP in water with those of Ib in micelles.