Films from copolyimides containing side carboxylic acid groups were prepared in -COOH and COO-Na+ forms to study the effect of the side groups content on some physical properties such as water absorption, thermal stability and electrical parameters. The thermal stability was evaluated by TGA and the results showed that degradation temperatures depended not only on the side groups content but also on the membrane ionic form. This dependence was also observed in the water sorption isotherms; the polyimides with the highest -COOH content showing the highest water sorption in the equilibrium. Dielectric impedance measurements revealed how the side groups and their ionic form affect the material electrical characteristics, in such a way that resistivity decreased and permittivity increased on introducing more -COOH groups in the polymer, which is also related to a higher water uptake of the films. (C) 2011 Elsevier B.V. All rights reserved.