We have used DSC, FTIR spectroscopy, and ac impedance techniques to investigate the interactions that occur within complexes of poly(vinylpyrrolidone-co-methyl methacrylate) (PVP-co-PMMA) and lithium perchlorate (LiClO4) as well as these systems' phase behavior and ionic conductivities. The presence of MMA moieties in the PVP-co-PMMA random copolymer has an inert diluent effect that reduces the degree of self-association of the PVP molecules and causes a negative deviation in the glass transition temperature (T,). In the binary LiClO4/PVP blends, the presence of a small amount of LiClO4 reduces the strong dipole-dipole interactions within PVP and leads to a lower T-g. Further addition of LiClO4 increases T-g as a result of ion-dipole interactions between LiClO4 and PVP. In LiClO4/PVP-co-PMMA blend systems, for which the three individual systems-the PVP-co-PMMA copolymer and the LiClO4/PVP and LiClO4/PMMA blends-are miscible at all compositional ratios, a phase-separated loop exists at certain compositions due to a complicated series of interactions among the LiClO4, PVP and PMMA units. The PMMA-rich component in the PVP-co-PMMA copolymer tends to be excluded, and this phenomenon results in phase separation. At a LiClO4 content of 20 wt% salt, the maximum ionic conductivity occurred for a LiClO4/VP57 blend (i.e., 57 mol% VP units in the PVP-co-PMMA copolymer). (c) 2007 Elsevier Ltd. All rights reserved.