Fluorescence decay measurements of the rate of non-radiative direct energy transfer have been employed to characterize the core and the core-corona interface of polyisoprene-poly(methyl methacrylate) (PI-PMMA) diblock copolymer micelles in acetonitrile. These micelles consist of a core of the insoluble PI blocks and a corona of the soluble PMMA blocks. The block copolymers are labeled, at the block junction, with a single fluorescent dye, either a donor chromophore (phenanthrene) or an acceptor chromophore (anthracene). Because the polymers are junction-labeled, the chromophores are naturally confined to the interface of each micelle. Analysis of fluorescence decay data indicate that energy transfer takes place on a flat spherical surface, which implies a strong segregation between PI and PMMA in the micelle. From the data analysis, a micellar core radius of (7.6 +/- 0.8) nm is calculated from which a number-average aggregation number of 98 +/- 22 is obtained. Static light scattering measurements give a weight-average aggregation number of 127 +/- 6. The PI-PMMA micelles are star-like with stretched PMMA corona blocks. The corona thickness of the micelles varies from 15.8 to 17.7 nm, using the hydrodynamic radius and core radius obtained from dynamic light scattering and fluorescence decay measurements, respectively.