The excited state reaction of a diphenyl methane dye, auramine O (AO), has been studied in water/AOT/heptane reverse micelles. When compared with bulk aqueous solution the reaction rate is strongly suppressed in the dispersed phase, and the kinetics are non-single exponential. The reaction rate is a strong function of micelle radius, increasing with increasing size. The results are discussed in terms of increased friction along the reactive coordinate due to confinement of the water phase and equilibrium between AO bound at the water/surfactant interface and in the dispersed water phase. (c) 2005 Elsevier B.V. All rights reserved.