The phase behavior of the ternary systems poly(ethylene oxide)/UCON/water and poly(propylene oxide)/UCON/water has been investigated experimentally and theoretically. UCON is a random copolymer of EO and PO segments. Both systems display closed immiscibility regions, provided that the composition of the UCON polymer is not too similar to that of the homopolymer. The immiscibility regions grow with increasing temperature due to worsening of the solvency of the polymers. The theoretical analysis is based on Flory-Huggins mean field lattice theory augmented with a model for polymers possessing internal states, and previously established parameters for EO and PO segments in aqueous solution are used. A good agreement between the experimental and the calculated results was obtained. Moreover, details about the effect of the balance of polymer-polymer and polymer-solvent interactions on the phase behavior are discussed.