Micelle formation in tetrahydrofuran-methanol, 1,2-dichloroethane-methanol, tetrahydrofuran-water, and water solutions by PS-PEO diblock copolymers was studied by the fluorescence spectroscopy. For this purpose, a series of PS-PEO copolymers was labeled by the phenanthrene or anthracene groups at the block junctions. The chromophore contents in copolymers were determined by the absorbance spectroscopy using suitable model compounds. When micelles were present, intramicellar nonradiative energy transfer could be observed. For the PS-PEO copolymer (M(n) = 114 000,93 weight % PEO) we find the onset of micelle formation in water in the range of 10 X 10(-3) g/L (9 X 10(-8) M).