The substitution reaction [Pd(bipy)(en)](2+) + en --> [Pd(en)(2)](2+) + bipy (where bipy = 2,2’-bipyridine and en = ethylenediamine) has been studied in water and water/AOT/n-heptane microemulsions at various values of the molar ratio R (R = [water]/[AOT]) by flow microcalorimetry. From calorimetric data, molar enthalpies of reaction and rate constants were obtained. These quantities indicate that, by increasing R, the reaction becomes less exothermic and its rate constant decreases, trending to the value observed in water. These features can be reasonably rationalized in terms of the peculiar solvation state of reactants inside the AOT reversed micelles and/or the peculiar physicochemical properties of micellar core.