Sulfonated polystyrene (s-PS) membrane has been tested for its electric conductivity using Impedance Bridge Type 1650B, degree of sulfonation using I:;TIR, and thermal stability using Thermogravimetric Analysis. The electric conductivity test shows that the present of sulfone grafted onto polystyrene (PS) increase conductivity. PS has electric conductivity 10-8 Stcm and SOps membrane has I O-~ S/cm on its electric conductivity. It is almost similar to conductivity electric of Nafion membrane as a commercial membrane for PEMFC. FTIR result shows that the change of electric conductivity is concerning with the extent of sulfonate preserce in grafting. Degree ofsulfonation was calculated from ratio of absorbance in 1180,3 cm-1 and absorbance in 1452,3 cm-l. It showed rising significantly beyond one hundred percent. Besides, sample mass decreased 3-12% by an isotherm for 5 hours at iO °C and 80 °C, and less than 5% by an isotherm at 120 °C for 5 hours too. Key word: electric conductivity, thermal stability, sulfonated polystyrene membrane, degree ofsulfonation. Indriyati, Sunit Hendrana dan Sri Pujiastuti; Pusat Penelitian Fisika (P2F) -LIPI