The nucleophilic aromatic substitution (SNAr) reaction of fluoride by hydroxide ion in 1-fluoro-2,4-dinitrobenzene has been studied in water in oil (W/O) microemulsion media. The microemulsions used were sodium bis(2-ethylhexyl)sulfosuccinate (AOT)–heptane–water or benzylhexadecyldimethylammonium chloride (BHDC)–benzene–water. The kinetic profile of the reaction was investigated as a function of variables such as (a) reactant concentrations, (b) surfactant structure and concentration and (c) amount of water dispersed in the microemulsion. The distribution constants (K) of the substrate between the micellar pseudo-phase and organic phase have been determined. They were used to evaluate the intrinsic second-order rate coefficient (kr) of the SNAr reaction in microemulsion solutions. In cationic W/O microemulsions the reaction is catalysed relative to homogeneous solutions, whereas in anionic W/O microemulsions the reaction is inhibited. A mechanism to rationalize the kinetic results is proposed.