Cu++ ion containing solid polymer electrolytes exhibit interesting electrochemical properties. In particular, the polymer electrolyte PEO9:Cu(CF3SO3)(2) made by complexing copper triflate (CuTf2) with PEO appears to show scientifically intriguing transport properties. Although some copper ion transport in these systems has been seen from plating stripping processes, the detailed mechanism of ionic transport and the species involved are yet to be established. In order to obtain enhanced ionic conductivities and also to contribute towards understanding the ionic transport process in Cu++ ion containing, PEO based composite polymer electrolytes, we have studied the system PEO9: CuTf2: Al2O3 incorporating 10 wt.% of alumina filler particles of grain size 10 mu m, 37 nm, 10-20 nm and also particles of pore size 5.8 nm. Thermal and electrical measurements show that the system remains amorphous down to room temperature. The composite electrolyte is predominantly an ionic conductor with electronic conductivity less than 2%. The triflate (CF3SO3-) anions appear to be the dominant carriers. The presence of alumina grains has enhanced the conductivity significantly from room temperature up to 100 degrees C. The nano-porous grains with 5.8 nm pore size and 150 m(2)/g specific surface area exhibited the maximum conductivity enhancement. This enhancement has been attributed to Lewis acid-base type surface interactions of ionic species with O2- and OH- groups on the filler grain surface. (C) 2005 Elsevier Ltd. All rights reserved.