Fluorescence energy-transfer measurements of labeled styrene-isoprene diblock copolymers were demonstrated to be sensitive to microphase separation or micelle formation in blends of the copolymers with polyisoprene. Anthracene and carbazole chromophores, serving as energy-transfer acceptors and donors, respectively, were attached to the terminus of the styrene block of the anionically polymerized copolymers. Microphase separation was shown to exist in blends of low molecular weight diblock copolymers with low molecular weight polyisoprene, based on changes in IA/ID, the ratio of acceptor to donor fluorescence intensities. However, the gradual increase exhibited in IA/ID with increasing block copolymer concentration indicates that a sharp transition, expected of a critical micelle concentration (cmc), is not apparent in these blends or that the cmc occurs at very low block copolymer concentration. The values of IA/ID obtained in systems with micelles were compared to a two-state energy-transfer model incorporating the presence of free block copolymer in the homogeneous matrix and block copolymer in micelles; for concentrations at which micelles exist, it was determined that the amount of free block copolymer increases as the total block copolymer concentration increases. The sensitivity of fluorescence lifetime measurements of the carbazole label to microphase separations in the blends was also investigated.