The electrical conductivity and thermal behaviour of solid polymer electrolyte films containing thulium triflate and poly(ethylene oxide) have been studied. Homogeneous electrolyte samples were prepared by using solvent casting and hot-pressing techniques to produce films with a composition defined by the general formula (EO)(n)Tm(CF3SO3)(3), where n lies between 3 and 150. This electrolyte system was found to show a surprisingly high total ionic conductivity in compositions with large salt concentrations. These materials, with n between 3 and 5, were apparently amorphous but crystallized slowly after prolonged annealing at room temperature. Thermal analysis showed the presence of a broad melting peak which may correspond to the fusion, or dissolution, of a salt-polymer complex.