Methyl orange dissolved in a water-in-oil microemulsion was examined by UV-VIS absorption spectroscopy and differential scanning calorimetry (DSC). Methyl orange showed two temperature-dependent peaks at 416 and 354 Mn in the microemulsion. The former was attributed to a methyl orange monomer, and the latter to aggregates of the dye in the parallel orientation (H-aggregates) at the interface between the water phase and the oil phase in the microemulsion. The position of methyl orange in the microemulsion was verified by DSC analysis.