The integrated TGA-FTi.r. technique probed the thermal degradation of : (1) a Nafion(R)-H+ membrane; (2) this membrane as modified by incorporation of a SiO2[1-x/4] (OH)(x) phase via in situ sol-gel reactions for tetraethoxysilane; and (3) this modified membrane as further modified, organically, via post-reaction with diethoxydimethylsilane. Gravimetric loss is multistep for (1) and (2), but occurs in a single step for (3) which has the greatest thermal degradative stability. The considerable inhibition of SO2 evolution for (2) and (3) is rationalized in terms of side-chains that are immobilized within silicon oxide cages that block reactions involving SO3 groups. Degradation of SiO2[1-x/4] (OH)(x) ＇cores＇ by generated HF is retarded by their organic ＇shells＇ in (3). Substituted carbonyl fluorides and CF2-containing fragments appear. Si-CH3 and CH3 groups and/or -CH=CH2 vinyl compounds are degradation products for (3). It is unlikely that intact R-SO2-OH groups, or perfluoroalkylether groups issuing from the side-chains of Nafion(R), exist as gas phase products. The origins of some peaks an in question owing to spectral complexity. Altogether, the hybrid (3) is superior with respect to degradation onset temperature, subsequent mass loss over a range of about 100 degrees C, and low quantity of evolved products.