The alkaline hydrolysis of sodium nitroprusside, [Fe(CN)5NO]2−, to yield [Fe(CN)5NO2]4− was studied in water in oil (w/o) AOT–isooctane–water microemulsions [AOT = sodium bis(2-ethylhexyl) sulphosuccinate]. Kinetic data show an initial increase of absorbance at 415 nm, due to the formation of [Fe(CN)5NO2]4−. Following this initial increase, the absorbance started to decrease after a period of time which depends on the particular experimental conditions and returned to the initial value. This anomalous behaviour was interpreted as a result of competitive AOT hydrolysis consuming HO− ions and regenerating [Fe(CN)5NO]2−. The influence of the microemulsion composition on the kinetic results was interpreted in terms of pseudophase formalism, assuming a uniform distribution of HO− and [Fe(CN)5NO]2− ions in the aqueous pseudophase. Analysis of the absorbance–time profiles enables us to obtain the second-order rate constants for hydrolysis of [Fe(CN)5NO]2− in the water droplet, (half that reported in pure water) and the second-order rate constant for hydrolysis of the surfactant (AOT), , in the microemulsion. Copyright © 2002 John Wiley & Sons, Ltd.