The effect of alumina additives bearing various surface groups on conductivity and lithium cation transference numbers in poly(ethylene oxide) dimethyl ether (PEO-DME)-LiClO4 electrolytes is examined. It is demonstrated that an increase in the conductivity and lithium transference number in composite electrolytes compared to pure PEO-DME-LiClO4 electrolyte is observed in the limited salt concentration range. Both quantities seem to depend mostly on ionic species mobility. Also, their salt concentration dependence resembles that of viscosity of electrolytes studied. The conduction mechanism is discussed on the basis of conductivity, transference numbers and ionic association studies. (c) 2005 Published by Elsevier Ltd.