Composite solid electrolytes 0.5LiClO(4)-0.5A with additives of nanocrystalline oxides A = alpha-Al2O3 gamma-Al2O3, alpha-LiAlO2, gamma-LiAlO2, were investigated by electrical measurements and Li-7 MNR spectroscopy. It was found that composites containing gamma-Al2O3 and gamma-LiAlO2 have close conductivity parameters (absolute values of conductivity, sigma, and activation energy, E-a); similar situation is observed in composites containing alpha-phases of Al2O3 and LiAlO2. Comparison shows that at the same specific surface area of the additive the composites containing gamma-phases have lower Ea values and higher conductivities than ones containing alpha-phases. The effect is suggested to be caused by the presence of tetrahedrally-coordinated aluminum cations in the structure of gamma-phases leading to the enhancement of their surface activity. Analysis of the temperature dependence of the second moment of Li-7 NMR spectra shows that at temperature higher than 400K the concentration of mobile lithium ions is comparable with the total content of LiClO4 in the composites. (C) 2008 Elsevier B.V. All rights reserved.