Methylene Blue (MB⁺) is a sensitizer that has been used for a variety of applications including energy conversion and photodynamic therapy (PDT). Although its photochemical properties in isotropic solution are well established, its effect in vivo and in restricted reaction environments is somewhat erratic. In order to understand its photochemical behavior when it interacts with biomolecules, in particular with membranes, MB⁺ properties were studied in sodium dodecyl sulfate (SDS) and cetyl trimethylammonium bromide (CTAB) solutions. Because of an electrostatic attraction, SDS and MB⁺ form complexes, changing the properties of both the micelles and the MB⁺ solutions. Surface tension measurements show that the c.m.c. of SDS decreases from ∼7 mM to ∼70 μM when the MB⁺ concentration increases from 0 to 45 μM. Above the c.m.c., binding of MB⁺ in the micelle pseudo-phase causes the formation of aggregates (mostly dimers) as attested by the increase in the absorption at 580 nm and the decrease in fluorescence emission. The extent of dimer formation is dependent on the relative concentrations of MB⁺ and SDS. In the presence of excess of SDS, MB⁺ is mainly in the monomer form and at low SDS concentration dimers are favored. Such effect, which was not observed in CTAB micelles, was modeled qualitatively by considering that MB⁺ molecules partition to the micelle pseudo-phase which favors or disfavors dimers as a function of its volume. MB⁺ transient species were characterized by laser flash photolysis and NIR emission showing the presence of triplets and subsequently singlet oxygen at high SDS concentration and semi-reduced and semi-oxidized MB⁺ radicals at low SDS concentration. Therefore it was shown that, depending on the ground state MB⁺ monomer/dimer equilibrium, induced by the micelles, the photochemical properties of MB⁺ can be shifted from a Type II (energy transfer to oxygen forming singlet oxygen) to a Type I mechanism (electron transfer forming the semi-reduced and the semi-oxidized radicals of MB⁺).