The phase behavior of triblock EO-PO-EO copolymer (Pluronic L(64))-homopolymer aqueous mixtures was investigated, and compared to that of EO-PO random copolymer (UCON)-homopolymer aqueous mixtures (EO refers to ethylene oxide and PO propylene oxide). Concerning the phase behavior of the isotropic solutions, no significant difference has been found between these two systems. The theoretical analysis is based on the Flory-Huggins mean field lattice theory for polymers possessing internal states, and previously established parameters for EO and PO segments in aqueous solution are used. The major difference between the block and the random copolymers is that the former can form, at finite concentrations, micelles and liquid crystalline mesophases in water. Addition of the PEO homopolymers destabilizes the mesophases, on the one hand, but may induce the mesophase formation, on the other.