The phase behavior of the three-component PEO-PPO-PEO/dextran/water system has been studied. Pluronic L64, P105, and F68 were used, and they are members of a family of triblock copolymers with the structure PEO-PPO-PEO, where PEO is poly(ethylene oxide) and PPO poly(propylene oxide). Moreover, these triblock copolymers tend to self-aggregate in aqueous solution at increasing temperature. The phase behavior of the three-component system was studied both experimentally and theoretically at different temperatures corresponding to the unimer (monomolecular) and the micellar region of the PEO-PPO-PEO polymers. The determination of the unimer and micellar regions was performed with a dye solubilization technique using 1,6-diphenyl-1,3,5-hexatriene (DPH) as a spectroscopic probe. A strong temperature effect on the phase behavior in PEO-PPO-PEO/dextran/water systems was observed in the case of Pluronic L64 and P105. At elevated temperature (40 degrees C) the micellization of the PEO-PPO-PEO polymers leads to a drastic change of the tie-line slopes as well as the shape of the binodals. Qualitative agreements were found between experimental findings and theoretical calculations using a Flory-Huggins lattice theory extended with internal degrees of freedom by modeling the PEO-PPO-PEO polymers as a poly(ethylene oxide) chain. This supports the prevailing picture that the more hydrophobic PPO block is shielded from water contacts by forming hydrophobic domains surrounded by EO segments.