Abstract: A polymeric electrolyte comprising a solution of an ionic salt in a polymer X having a bacbone of formula --A--B--, where A is an oligomeric chain and B is a chain extender group, attached to oligomer A, containing one or more linkage atoms Z selected from Si, C and P, wherein the polymer contains oxymethylene units which are present in one or more of the polymer backbone, sidechains or crosslinks, such that where A is an oxyethylene oligomer of formula --CH.sub.2 CH.sub.2 --O).sub.n, n is an integer from 3 to 10 and where the oxyethylene oligomer is contained in a sidechain or crosslink attached at B, n is from 2 to 25.