With an objective to assess the suitability of the dry proton conductor cesium hydrogen sulfate (CsHSO4) as an electrolyte for fuel cells, its chemical stability in H-2 atmosphere in presence of the electrocatalyst Pt/C has been investigated. X-ray diffraction and differential scanning calorimetric (DSC) data indicate that CsHSO4 decomposes to CS2SO4 and H2S at 150 degreesC in H-2 atmosphere when it is mixed with Pt/C catalyst, while it is quite stable under identical conditions in the absence of the Pt/C catalyst. Although thin composite membranes (similar to80 mum) prepared with poly(vinylidene fluoride) and CsHSO4 exhibit high proton conductivity at 150-200 degreesC that is adequate for fuel cell applications, development of compatible non-platinum alloy or transition metal oxide catalysts is needed before CsHSO4 could be considered as an electrolyte for fuel cells. Also, processing procedures need to be optimized to obtain dense, gas impermeable membranes suitable for fuel cell applications. (C) 2003 Elsevier Science Ltd. All rights reserved.