The effects of urea on the micellization properties of a poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) copolymer (commercially available as Pluronic P105) and on the structure and microenvironment of the micelles are reported. Critical micellization concentration (cmc) and temperature (cmt) values for the amphiphilic block copolymer dissolved in urea/water mixtures (urea concentration 0, 1, 2, and 4 M) were obtained using a dye solubilization method and corroborated with data from surface tension, density, and fluorescence spectroscopy experiments. Urea increased the cmc and cmt of the PEO-PPO-PEO copolymer; the effect of urea on the cmt was more pronounced at low copolymer concentrations and diminished at concentrations of similar to 2.5%. The thermodynamic parameters of micelle formation in the presence of urea were estimated using a closed association model; the enthalpy of micellization was positive (endothermic) and decreased upon increasing the urea concentration. The surface activity and the partial specific volume of the block copolymer decreased with an increase in the urea concentration, whereas the hydrodynamic radii of the copolymer micelles, determined using dynamic light scattering, remained unaffected by the presence of 4 M urea in the solution. The micropolarity in copolymer solutions in urea/water 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; a small decrease in the micropolarity of the micelle core was observed in the presence of urea. The microviscosity in the micelle interior, estimated from the intramolecular excimer fluorescence of the hydrophobic probe bis(1-pyrenylmethyl) ether (dipyme), also exhibited a small decrease with an increase in the urea concentration. The findings presented here are discussed in the context of the molecular mechanism underlying the effects of urea.