The electrical and thermal studies have been carried out for nonaqueous proton conducting gel-electrolytes based on commercially available poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP) system. The electrolytes consist of a PVdF-HFP matrix and entrapped solvents such as propylene carbonate (PC), ethylene carbonate (EC), dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) with a dissolved phosphoric acid. DSC results show that excess phosphoric acid crystallizes for DMF and DMSO-based gel electrolytes and the compatibility of PC and DMF with polymer host is better than EC and DMSO. However, the compatibility is remarkably improved with proper annealing treatment. For all gel systems, the maximum in the proton conductivity is achieved in the range between 10 and 20 wt.% of H3PO4. The proton conductivity of EC and PC-based electrolytes is higher than DMF and DMSO-based electrolytes, possibly because of higher acid dissociation rate due to relatively higher dielectric constants of EC and PC. (C) 2004 Elsevier Ltd. All rights reserved.