The decomposition reaction of the purple dye murexide in acidic media is used as a probe indicator for protons in nonionic microemulsions The reaction kinetics primarily rely on the proton concentration and permit assessment of the proton activity in the nonionic microemulsions of water/cyclohexane/Igepal and water/heptane/Igepal The experiments performed in the two microemulsions covered a wide range of water-to-oil mass fraction for the two systems The kinetic runs were monitored under pseudo-first order conditions by the stopped-flow technique The equilibrium constants for the formation of purpuric acid and the kinetic constants for the ensuing decomposition reaction fulfill a trend consistent with the micro compartmentalized nature of the multicomponent medium and support the use of murexide as an indicator of the proton activity in microemulsions (C) 2010 Elsevier Inc All rights reserved