The effects of temperature on the micellization properties and the structure of the micelles for two poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) copolymers, having similar size hydrophobic (PPO) blocks and different size hydrophilic (PEG) blocks, are reported. Critical micellization concentration (CMC) and temperature (CMT) values for aqueous copolymer solutions were obtained from a dye solubilization method and corroborated with differential scanning calorimetry, surface tension, density, light scattering intensity, and fluorescence spectroscopy experiments; very good agreement among the different techniques is observed. The extraction of CMC and CMT values from the characteristic features of the data obtained from the various techniques is discussed. The hydrodynamic radii of the copolymer micelles, determined using dynamic light scattering, remained constant over the temperature range investigated; the polydispersity of the micelle size decreased with temperature. The micropolarity in aqueous copolymer solutions was probed as a function of temperature using the I-1/I-3 intensity ratio of the pyrene vibrational fine structure recorded in fluorescence emission spectra. The decreasing values for the intensity ratio, following the formation of micelles, were considered to be a linear combination of the temperature effects on polarity observed in bulk PEO and PPO homopolymers, and in water. The composition of the micelle core was estimated using these data, indicating little or no penetration of water and approximately 25% PEO content.