The temperature variation of the I-1/I-3 ratio of pyrene vibrational fine structure has been studied in aqueous poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) copolymer solutions, in bulk PEG, PPO, and various organic solvents. Linear decreases were observed in organic solvents and in mixtures of bulk PEO and PPO. The data in aqueous PEO-PPO-PEO copolymer solutions reveal three distinct solution conditions : at low temperatures, the linear decrease with temperature parallels that for water; over a small intermediate temperature range there is a sharp decrease in the ratio, attributed to the formation of micelles; and at higher temperatures, a linear decrease is again observed. Values in this region are strongly dependent on the PPO block and are consistent with the representation of micelles with a core dominated by PPO. These decreasing values for the intensity ratio in the latter region indicate a less polar environment for pyrene and were considered to be a linear combination of the temperature effects on the polarity observed in bulk poly(ethylene oxide) and poly(propylene oxide) homopolymers. The composition of the micelle core was estimated using these data and indicated an increase of poly(ethylene oxide) as the polymer becomes more hydrophilic. In addition, the size of the micelles and their aggregation numbers were determined using light scattering and pyrene luminescence decay studies, respectively. These data showed an increase of the aggregation number with temperature while the hydrodynamic radius remained constant and were interpreted in terms of dehydration of the PEO block.