Replacing the counterion in sodium bis(2-ethylhexyl)sulfosuccinate (NaOT, usually known as AOT or Aerosol OT) with H+ (HOT) allows strongly acidic microemulsions to be obtained through the effect of a change in the solvation mechanism of the surfactant, where the Na+center dot center dot center dot OH2 interaction is displaced by a stronger H+center dot center dot center dot OH2 interaction. This raises the proportion of water bound to the counterion, which is reflected in the FT-IR spectrum for water trapped in the microemulsion and the H-1 NMR spectrum for the hydrogen atoms in the water molecules. In NaOT microemulsions, the resonance signal for hydrogen atoms in the water molecules increases from delta approximate to 3.9 ppm at W = 2 (with W = [H2O]/[NaOT]) to delta approximate to 4.8 ppm at W = 50. In HOT microemulsions, the disparate strength of Na+center dot center dot center dot OH2 and H+center dot center dot center dot OH2 interactions results in a decrease in the resonance signal for the hydrogen atoms in the water molecules from delta approximate to 8.6 ppm at W = 2 to delta approximate to 4.9 ppm at W = 50. These changes in the physical properties of water alter chemical reactivity in a way that is clearly apparent in solvolytic processes in NaOT and HOT microemulsions. Thus, the rate constants of reactions involving an associative mechanism increase with decreasing W in NaOT microemulsions, but decrease with decreasing W in HOT microemulsions. The disparate behavior is a result of a decreased nucleophilicity of interfacial water in HOT microemulsions relative to NaOT microemulsions. For a dissociative process the rate constants are greater in HOT microemulsions than in NaOT ones, and increase with increasing W in both types of microemulsions, which can be ascribed to an increased electrophilicity of interfacial water in HOT microemulsions.