The crystal structure of the polymer electrolyte poly(ethylene oxide)(4):RbSCN has been elucidated by powder X-ray diffraction crystallography. The structure is important since it is the largest cation system to be established to date. The complex is isostructural with the previously characterised 4:1 complexes of the salts KSCN and NH4SCN. It consists of the PEO chain in helical conformation enclosing the large Rb+ cation in each turn of the helix. Rb+ is coordinated by five ether oxygens and two SCN- ions. The thiocyanate anions in turn coordinate two neighbouring cations simultaneously along one PEO chain. As in all the previously determined 3:1 and 4:1 PEG-salt complexes, there are no direct interchain interactions. The general structural principles which may be determined from the crystal structures solved so far are discussed including the effect of cation size (ranging from Li+ to Rb+) on the PEG conformation.