The poly(ethylene oxide)-urea binary system forms a stable (alpha) complex that can be melt-quenched into a mixture of urea and a second (beta) complex with a different stoichiometry. Our results confirm that this mixture is metastable, but that the beta-alpha solid-solid transition is much slower than previously reported. In contrast with the melt-quenched mixture, we observe for the first time that the pure complex, prepared by electrospinning. is thermodynamically stable up to its melting temperature. This melting is incongruent and leads to the formation of the crystalline cc complex and liquid PEO. A phase diagram is drawn over the complete composition range and allows interpreting the formation of the various out-of-equilibrium mixtures observed experimentally. This work also demonstrates an interesting new possibility of electrospinning: the direct preparation of thermodynamically stable compounds that are otherwise inaccessible because of the formation of kinetically frozen products. (C) 2009 Elsevier Ltd. All rights reserved