The thermally-induced transitions of a water-soluble block copolymer, poly(ethylene oxide)-poly(propylene oxide) (MW 12 000, PO/EO ratio 3.46:1), were investigated in mixed aqueous-organic solvents using a high-sensitivity differential scanning calorimeter(DSC). The endothermic transitions of the block copolymer are markedly influenced by the addition of various organic cosolvents, ethylene glycol (EG), enthanolamine (EA), ethylenediamine (ED), 8-methoxyethanol (2-ME), and 1-propanol (1-PrOH). The transition temperatures, T-m, are lowered from similar to 30 degrees C in water to similar to 10 degrees C in the mixed solvents at relatively low cosolvent mole fraction (X less than or equal to 0.15); the transition enthalpies, Delta H-m, are simultaneously decreased. Among the five cosolvents, 1-PrOH exhibits the largest effect on T-m and Delta H-m, whereas ED displays an effect on T-m similar to that of 1-PrOH but the weakest effect on Delta H-m. T-m versus Delta H-m may be described by a second-order polynomial with a linear term (rho(1)) and a nonlinear term (rho(2)) Since T-m variations relate to solvent composition changes, rho(1) and rho(2) may be viewed as reflecting respectively the cosolvent/water interactions or affinities and changes in these interactions in the presence of the copolymer (ternary cosolvent/water/copolymer interactions). Attempts to correlate the rho(1) values with various parameters of the cosolvents show a qualitative relationship between rho(1) and a solvent hydrophobic parameter (theta) derived from the binary water/cosolvent mixture using the Kirkwood-Buff theory. The trends observed in the transition enthalpies (Delta H-m) and entropies (Delta S-m) are consistent with the view that the transition is initially of the type sol-micelle and that the hydrophobic effects associated with the PO segment of the copolymer, or with the cosolvent, largely dominate this process.