Conductivity and thermal properties on (PEO)(16)LiClO4 complexes mixed with various inert fillers have been studied. Among 22 fillers studied, a composite material with truely exceptional conductivity has not been found. As the inert particles are added, the glass-transition temperature decreases indicating higher flexibility of PEO chain; at the same time the heat of melting increases indicating higher crystallinity of polymer network, which reveals that the ceramic particles act as nucleation centers. The polymer-ceramic grain boundaries are found not to contribute appreciably to ionic conduction.