In acetonitrile, polyisoprene-b-poly(methyl methacrylate) (PI-PMMA) diblock copolymers form starlike micelles with a dense core of the insoluble PI blocks and a soft solvent-swollen corona of the soluble PMMA blocks. Static and dynamic light scattering experiments in combination with viscosity measurements show that these micelles behave hydrodynamically as hard spheres. The block copolymers are labeled at the block junction, with a single fluorescent dye, either a donor chromophore (phenanthrene) or an acceptor chromophore (anthracene). These dyes are confined to the interface during self-assembly. Fluorescence energy-transfer experiments on molecularly mixed micelles of donor-and acceptor-labeled copolymers provide a core radius of 7.6 (0.8 nm and a number-average aggregation number (N n agg) of 98 (22 under the assumption that the energy transfer takes place on a surface of a sphere. Simulations in terms of a Helfand-Tagami junction distribution profile confirm that the core-corona interface of the PI-PMMA micelles is thin (ca. 0.9 nm) and that almost all of the energy transfer occurs within a narrow interfacial region. From the static light scattering measurements of the mixed micelles a weight-average aggregation number (N w agg) of 127 (6 is obtained. The ratio N w agg /N n agg) 1.3 agrees with size polydispersity of the micelles obtained from the analysis of dynamic light scattering data. The experimental corona thicknesses are in good agreement with those calculated from expressions describing starlike block copolymer micelles.