Ionic conductivity and thermal properties on (PEO)(16)LiClO4 complexes mixed with 29 different inert fillers have been studied. The ionic conductivity at 100 degrees C, where the polymer electrolytes are mostly amorphous, does not change markedly, revealing that ceramic particles are almost inert. At room temperature, the ionic conductivity was more or less enhanced or reduced according to filler species, indicating that the ceramic particles give rise to morphological changes, thus modifying the crystallinity and PEO chain stiffness. We have examined whether any relations exist between various quantities such as the conductivity, particle size, glass transition temperature (T-g), melting temperature (T-m) of PEG, and heat of melting. For 30 different species, we found a correlation only between T-g and T-m. Among various factors, the size and morphology of spherulite and nonspherulitic amorphous materials were regarded to be of primary importance on the occurrence of the correlation. (C) 2000 Elsevier Science Ltd. All rights reserved.