Block copolymers of poly(N-decylacrylamide-b-N,N-diethylacrylamide) (PDcA-b-PDEA), with differentPDEAblock lengths and a constantPDcAblock labeled with a phenanthrene fluorescent dye at the PDcA R-chain-end were prepared by RAFT polymerization. These copolymers form star-like micelles in water, (critical micelle concentration below 0.1 g/L, determined using coumarine 153) with a PDcA insoluble core surrounded by a PDEA corona showing thermoresponsive properties. The kinetics of Forster resonance energy transfer (FRET) between the chain-end phenanthrene groups and anthracene loaded into the hydrophobic core of the micelles in water, was analyzed using a new model for energy transfer in spherical nanodomains. This model takes into account the Poisson distribution of the acceptors in the micelle population and the existence of two phenanthrene states with different fluorescence lifetimes. The analysis yields the radius of the micelle core, Rc=2.7±0.1 nm, with no need for deuteration of the core block. The result is compared with the value obtained by extrapolation of the light scattering data using the star micelle model, Rc(DLS)=3.0 nm. Themodel for star-likemicelles also yields a solvent-corona interaction parameter that changes with temperature due to the thermoresponsive nature of PDEA.