The thermal degradation of radiation-grafted and sulfonated FEP-g-polystyrene membrane has been investigated in the temperature range 50-650 degrees C in an inert gas atmosphere by thermogravimetry. The evolved gaseous products were analyzed by FTIR in combination with mass spectrometry to correlate weight losses at different temperatures with molecular composition of the copolymer membrane. On the basis of these results, a simple degradation model has been established.