The electrochemical, thermal and structural properties of polyethylene oxide (PEO) based polymer electrolytes containing multivalent ions were investigated. The phase diagram for the PEO:Ca(CF3SO3)2 system was determined by x-ray diffraction and differential scanning calorimetry techniques. Precipitation of the salt from the system at high temperatures was directly observed by variable temperature x- ray diffraction. This was ascribed to a negative entropy of dissolution of the salt in the polymer. A new crystalline complex PEO6Ca(CF3SO3)2, which exhibits a phase transition between two polymorphic forms was observed. The temperature dependence of ionic conductivity was related to the phase diagram. Redox behaviour of the PEO:Nil2 system was probed. Motion of the Ni(II) species through the system was extremely slow as evidenced by the low effective diffusion coefficient (1.82 x 10 11 cm 2s−1) and cationic current fraction (F+ < 0.1). Deposition of nickel from the polymer was characterised by instantaneous nucleation followed by three dimensional diffusion controlled growth. Investigation of the redox behaviour of the PE0:Eu(CF3SO3)3 system indicated that reduction of Eu3+ followed an ec mechanism. Evidence was obtained for extremely slow diffusion of Eu3+ containing species (D[sub]eff ~ 3.66 x 10 −16cm2s−1) through the system and slow kinetics of electron transfer. Thermal studies of the PEO:Co(SCN)2 system indicated that the glass transition temperature (Tg) was grossly elevated by the presence of Co(SCN)2 in the polymer. The absence of a crystalline PEO:Co(SCN)2 complex was ascribed to the high Tg which leads to slow crystallisation kinetics. UV-visible spectra indicated that the Co2+ ion was tetrahedrally coordinated in the system at low salt concentrations. The structure of the PEO3NaClO4 crystalline complex was reported as a subsidiary study.