The structure and dynamics of a prototype polymer electrolyte, PPO-LiClO4, have been investigated using neutron diffraction (ND) and quasi-elastic neutron scattering (QENS). For comparison, corresponding studies of pure PPO have also been performed. The diffraction data reveal large structural changes which are induced by the dopant salt. The phenomena can be explained by local ordering of the chain segments around the solvated cations and by contraction of neighbouring chains via cationic cross links. The QENS results indicate that the segmental motions of the polymer chains, which have been considered to be of vital importance for the ionic mobility, slow down considerably when coordinated to cations. The results are discussed in relation to proposed models for the structure and the conductivity mechanism. (C) 1998 Published by Elsevier Science B.V. All rights reserved.