In situ UV-vis spectroscopy has been applied to study the hydrolysis of acetylsalicylic acid, as a model reaction, in sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/supercritical ethane reverse micelles in the presence of imidazole as a catalyst. A 55-fold increase in the rate constant was observed in AOT/supercritical ethane micelles compared to the reaction carried out in aqueous buffer medium. The promotion of hydrolysis in the micelles as compared to aqueous solution has originated from compartmentalization of water-soluble reactant and the catalyst in the aqueous core of micelles.