Observed phases and transition temperatures in mixtures of poly(ethylene oxide) and sodium thiocyanate (PEO–NaSCN) are described by a binary phase diagram. The components of this system are semicrystalline PEO which can dissolve salt up to a mole fractionXs=0.023, and a crystalline complex of composition PEO3.5NaSCN (Xcc=0.22). The salt‐deficient PEO component is normally formed by a eutectic reaction with eutectic temperatureTe=63 °C. Conductivity of PEO–NaSCN mixtures is calculated assuming that only ions in the amorphous, liquidlike regions contribute to charge transport. This model predicts a positive discontinuity in conductivity on eutectic melting, and a sharp maximum in conductivity at low salt concentrations, in agreement with observed behavior.