The enhancement of the ionic conductivity of Li-P(EO)(n)-based polymer electrolytes by the addition of finely divided inorganic oxides is the subject of considerable discussion. The increase in ionic conductivity in concentrated composite solid polymer electrolytes (CSPE) is related both to the suppression of the formation of crystalline PEO and PEO-salt phases and to interfacial conduction. In the case of polycrystalline or polyparticle solid electrolytes, however, grain-boundary (GB) resistance, which is associated with the crossover of ions from particle to particle across grain boundaries orthogonal to the direction of current flow (or parallel to the field lines), must be considered. The conduction across grain boundaries in polymer appears to be ignored so far. In this work, we provide SEM and ECS experimental data on LiI-P(EO)-Al2O3 CSPEs. The effects of plasticizers, doping by CaI2, change in Li/EO ratio, and nanosize alumina concentration on the interior grain ionic conductivity and on the resistance of the orthogonal grain boundaries are addressed.