A series of solid polymer electrolytes prepared from commercial poly(ethylene oxide) and lanthanide triflates (La3+, Nd3+ and Eu3+), in which the salt ions are solubilised by complexation with the oxygen atoms in the flexible polymer molecule, has been studied using ac conductivity and thermal analysis measurements. Two techniques, solvent casting and hot-pressing, were applied to produce a range of polymer electrolyte compositions. The differences in the behaviour of electrolytes produced using these two preparative techniques have been justified in terms of partial decomposition of the electrolytes, the effects of chain entanglement and the presence of residual solvent molecules increasing the free volume of the polymer chains.