The phase behavior of aqueous solutions of polymers containing ethylene oxide (EO) and propylene oxide (PO) has been modeled on the basis of a mean-field lattice theory for multicomponent mixtures of copolymers with internal states occurring in heterogeneous systems. The regions for monomeric solution, spherical micelles, and elongated rods have been examined for three (EO)(x)(PO)(y)(EO)(x) triblock copolymers. A semiquantitative description of the strong temperature dependence of the phase behavior was obtained. At low polymer concentrations and at low temperatures a monomeric solution was found, whereas at either higher temperatures or higher concentrations a solution of spherical aggregates was present. At even higher temperatures, there is a transition from spherical to infinitely long rodlike aggregates, and eventually the system separates into two phases. Segment density profiles indicate that, at a given concentration, the radial extension increases whereas the headgroup area decreases on increasing temperature for both micelles and rods. At the transition from micelles to rods the radial extension reduces abruptly by ca. 10%.