The structure and conductivity of mixtures of poly(ethylene oxide) (PEO) with a molecular weight (MW) of 4000 000 and KSCN vary with salt concentrations. The PEO-KSCN material of a molar ratio ethylene oxide unit to cation (O/K+) of 16/1 has low conductivity, and its crystal structure is similar to that of pure PEG, but with a lower crystallinity and melting point. The 8/1 material shows a high conductivity and exhibits an almost completely amorphous state, but the 4/1 material has a very low conductivity and a completely different crystal structure with a very high melting temperature. Mixed salt (LiSCN-NaClO4) systems increase the conductivity. It is suggested that these results can be explained in terms of molecular disorder in the PEG-salt phase, configurational entropy and/or degree of salt dissociation. Mixed anions (KSCN-KI) can also increase conductivity in systems having a high proportion of the SCN- ion. PEOs with a MW of 600 and 18 500 show a possible application as plasticizers to increase conductivity, due, in part, to the higher content of hydroxyl end-groups to dissociate the salt into free ions and, in part, to the lower glass transition temperature (T-g) PEO (MW 600) gives a better result than PEO (MW18 500) at the same proportion in the two MW systems. A blend of three molecular weights (4 000 000, 18 500 and 600) has a higher conductivity and a lower crystallinity than do blends of two molecular weights. These phenomena are discussed in terms of random coil behaviour or the lack of it for the lower molecular weight species.