The chemical and thermal stability of several solid acid compounds under fuel cell operating conditions has been investigated, primarily by thermogravimetric methods. Thermal decomposition of CsHSO4, a material which has shown promise as an alternative electrolyte for proton exchange membrane (PEM) fuel cells, initiates at similar to175degreesC under inert conditions. The overall decomposition process can be expressed as 2CsHSO(4) --> CS2SO4 + H2O + SO3 with CS2S2O7 appearing as an intermediate byproduct at slow heating rates. Under reducing conditions, chemical decomposition can occur via reaction with hydrogen according to 2CsHSO(4) + 4H(2) --> CS2SO4 + 4H(2)O + H2S. In the absence of fuel cell catalysts, this reduction reaction is slows however, materials such as Pt, Pd, and WC are highly effective in catalyzing the reduction of sulfur and the generation of H2S. In the case of M3H(XO4)(2) compounds, where M = Cs, NH4, or Rb and X = S or Se, a similar reduction reaction occurs: 2M(3)H(XO4)(2) + 4H(2) --> 3M(2)XO(4) + 4H(2)O + H2X. In an operational fuel cell based on CsHSO4, performance degraded with time, presumably as a result of H2S poisoning of the anode catalyst. The performance loss was recoverable by exposure of the fuel cell to air at 160 degreesC.