Dehydroergosterol (DHE) is synthesized and investigated as a fluorescent analog of cholesterol to examine the micelle-nonionic surfactant vesicle (NSV) transition induced by octyl glucoside (OG). NSV bilayer is based on diglycerol hexadecylether/cholesterol/dicetylphosphate (47.5/47.5/5 wt%) admixture containing less than 1.5 mol% of DHE. Lipids-OG mixed aggregates, during vesicle solubilization by detergent or reconstitution through dilution with detergent-free buffer or dialysis of mixed micelles, are characterized by quasi-elastic light scattering and time-resolved fluorescence. Emission intensity decays of DHE at 371 nm (excitation at 327 nm) are obtained by the time-correlated single-photon-counting technique and analyzed by the maximum entropy method. The excited-state distribution of DHE is presented as a function of the detergent composition of the mixed lipids-OG aggregates. The data unambiguously show two well-resolved lifetime classes at each stage of the transition, if the samples are in the equilibrium state. The vesicular domain exhibits a major long lifetime component and a second shorter one, probably reflecting difference in molecular order. Significantly faster decay parameters are observed in the micellar domain, corresponding to the disappearance of the long-lived excited state. In between, the coexistence of bilayered and micellar structures is depicted by both the persistency of the long lifetime component and the enrichment of the short lifetime population. The kinetics of NSV micellization and formation appear strongly different at a molecular level, as evidence of the vesicle membrane impermeability.