The use of lithium cation in composites of block copolymers [polyethylene-b-polyethylene oxide (PE-b-50%PEO and PE-b-80%PEO)] and their derivatives was tested as a modifier of the vapor sorption and impedance of these complexes. The block copolymer PE-b-80%PEO was modified by oxidation of its hydroxyl end group to both a carboxylic acid group (PE-b-80%PEO)CH2COOH and its sodium salt (PE-b-80%PEO)CH2COO- Na+ for the purpose of improving its compatibility and performance as a matrix for composites. These modified copolymers were characterized by FTIR, DSC, and mass spectrometry. The sorption of water of these copolymers and their composites with lithium nitrate was also compared, as well as the electrical properties of their composites were measured by electrical impedance spectroscopy. For the composites obtained with PE-b-80%PEO and lithium nitrate, it was found that lithium cation plays an important role increasing the sorption rate, which is maximized for the PE-b-80%PEO + (21% lithium nitrate) composite. For the copolymers (PE-b-80%PEO)CH2COOH and (PE-b-80%PEO)CH2COO- Na+ and their composites, the highest sorption rate was observed for salt in the following order: COO Na+ > COOH > OH. The PE-b-80%PEO + (21% lithium nitrate) composite behaves as a solid polymeric ionic conductor fitting the Williams-Landel-Ferry equation. However, both (PE-b-80%PEO)CH2COOH and (PE-b-80%PEO)CH2COO- Na+ (21% lithium nitrate) composites fitted the Variable Range Hopping equation, indicating a conductance trend with temperature governed by a thermally activated with energy of 0.482 and 0.524 eV and not by a relaxation process. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1809-1817, 2010