It has been previously shown that the addition of surface-modified Al2O3 results in the enhancement of conductivity for systems based on polytethylene glycol)-alkali metal complexes. The filler Al2O3 surfaces are neutral, acidic or basic. This effect, however, is not observed in cases where the electrolytes are based on a dimethyl-capped PEO matrix due to the absence of the terminal OH groups. The effects of the filler surface groups on the conductivity are explained by the interactions between filler surface groups, salt and polymer chains, and confirmed by the changes in ionic associations of the composites. The importance of the role of the polymer terminal end groups in the composite electrolytes has been highlighted. (C) 2004 Elsevier B.V. All rights reserved.