Polymers containing rare earth metal salts are potentially interesting laser materials, where it is most important that the materials should have good optical quality. In this connection, comparative studies of the relationship between ion pair/clustering formation and cation-PEO coordination have been made for the system Ln(CF3SO3)3PEOn for different concentrations and temperatures, where Ln=La, Nd, Eu, Dy and Yb and n=9, 16 and 24. The nu(COC) modes in PEO and the nu(SO3) modes in the CF3SO3- ion have been monitored using FTIR spectroscopy. All samples were totally amorphous for n=9 and 16, while a mixture of crystalline PEO and an amorphous phase was obtained for n=24. Different ion aggregates (pairs/clusters) involving cations and the triflate ions have been observed, as evidenced by splittings and shifts in the symmetric and the antisymmetric SO3 stretching bands. Shifts in the nu(COC) modes indicate bonding between cations and ether oxygens. The spectra at any given concentration and temperature are almost identical for all rare earth metals studied except ytterbium, suggesting an isomorphous structure.