Abstract In this work we report the preparation of membranes based on poly (vinyl alcohol) (PVAL), lithium hydroxide, alumina, and water with different alumina concentrations. The samples were characterized by impedance spectroscopy, differential scanning calorimetry (DSC), thermogravimetry, and x‐ray diffraction. The DSC thermograms show three anomalies after the first thermal treatment, the first one as a downward jump at around 20 °C, associated to the glass transition of the polymer, the second one as an endothermic peak around 210 °C associated to the polymer melting point, and the third one as an endothermic peak around 235 °C associated to the decomposition process. The results show that the glass transition temperature is strongly sensitive to the alumina content. Impedance results show a dielectric relaxation that disappears with increasing alumina concentration. The X‐ray diffraction shows that the amorphous part of the composite increases as the alumina concentration increases, indicating an approximation effect of the dispersed ceramic nanoparticles in the polymer matrix. These results can be interpreted by considering the Hodge et al. criterion, which establishes a correlation between the height of this peak an the degree of crystallinity of PVAL. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)