Terminal hydroxyl groups of micellized dodecyl (10) and (23) polyoxyethylene glycol (C12E10 and C12E23, respectively) are deprotonated by OH-, and the alkoxide ions react with 2,4-dinitrochlorobenzene (DNCB) giving ethers, but do not react with p-nitrophenyl dimethyl and diethyl phosphate (pNPDMP and pNPDEP, respectively), which react wholly with OH-. These substrates have similar hydrophobicities and should locate similarly in the micellar interfacial regions. These differences in reactivities are due to differences in relative nucleophilicities of OH- and alkoxide ions in aromatic nucleophilic substitution and dephosphorylation in micelles. Reactivities of DNCB and the related ethers indicate that local concentrations of OH- are similar in the aqueous and micellar interfacial regions but those of the alkoxide ions are higher in C12E10 than in C12E23 micelles due to differences in the molar volumes of the interfacial regions. With time the initially formed ethers react at similar rates with OH-generating 2,4-dinitrophenoxide ion.