The effect of filler surface group on the conductivity, ion-ion, ion-polymer interactions, and microstructure of PEG-LiClO4-Al2O3 composite polymer electrolytes is studied. It is shown that the addition of fillers results in an increase in ionic conductivity of polyether electrolytes observed in the narrow lithium salt concentration range. The position of conductivity maximum depends on the type of the surface groups of the filler and results from the Lewis acid-base type interactions between filler surface centers, ions, and ether oxygen base groups. These interactions an reflected by changes in the polymer chain flexibility observed by DSC and rheological experiments as well as microstructural changes due to polymer-filler-Li+ interactions as revealed by FT-IR experiments. Finally, the addition of the filler results in the changes in ionic associations as studied by FT-IR and by applying the Fuoss-Kraus formalism to the salt concentration dependence of the molar conductivity of the composite electrolytes studied.