Two and three-phase microemulsion systems have been investigated as new potential reaction media for the "dark" i.e. non photochemical, singlet oxygenation of organic substrates by the molybdate-catalyzed disproportionation of hydrogen peroxide. Examination of the phase behaviour of various oil/water/alcohol/surfactant/sodium molybdate systems has been carried out to reveal the boundaries separating the different Winsor systems. Comparative peroxidations of a model substrate in the multiphase systems showed surprising results with regard to the influence of the microemulsion microstructure and of the water excess phase. The latter dramatically favors the O-1(2) quenching by the solvent. The influence of hydrogen peroxide addition on the substrate conversion has also been investigated, showing decreasing performances as the reaction medium is diluted. Successful oxidations of alpha-terpinene and beta-citronellol have been carried out in a two-phase system (Winsor 1) with similar efficiency but with much simpler work-up, of the reaction media than in monophasic microemulsions. (c) 2006 Elsevier B.V. All rights reserved.