The solution structure of a representative polyether block copolymer (Pluronic P105: EO37-PO58EO37, where EO and PO denote ethylene oxide and propylene oxide segments, respectively) has been investigated in mixed solvents (selective for the PEO block) consisting of water and one of the following polar cosolvents: formamide, ethanol, and glycerol. EO37PO58EO37 self-assembles to form micelles in all the cosolvent-water mixed solvents examined, above a certain concentration (cmc) and temperature (cmt) which depend on the cosolvent type and content. Micelle structural parameters were obtained as a function of the cosolvent-water ratio, solution temperature, and block copolymer concentration by fitting a core-corona form factor and a hard-sphere interaction structure factor to small-angle neutron scattering (SANS) data. The micelle radii, micelle core radii, micelle association numbers, and polymer volume fractions in the micelle core and corona decreased with increasing cosolvent-water ratio in the mixed solvents containing formamide or ethanol. Addition of glycerol led to higher micelle association number and higher polymer volume fraction in the micelle corona. The above findings are discussed in terms of the solvent quality in the mixed solvents. An increase in temperature resulted in higher micelle association number and a lower degree of solvation in the micelle core and corona in all the cosolvent-water mixed solvents examined.