In this study, poly(styrene-isobutylene-styrene) (SIBS) block copolymers were characterized by thermogravimetry as a function of sulfornation level (53-97%) and counter-ion substitution (Mg2+, Ca2+, Ba2+). Sulfonated samples showed an additional minor loss of mass at approximately 290 +/- 2 degrees C, which was not observed in the unsulfornated polymer. At this temperature, desulfonation or a cleavage reaction of the aromatic carbon-sulfur bond occurs. The counter-ion substituted membranes did not show the degradation temperature at 290 degrees C, but additional unique degradation temperatures above 500 degrees C, suggesting improved thermal stability for the ionically cross-linked polymer with cations. Some cations (Mg-2+,Mg- Ca2+, Ba2+), showed multiple high temperature degradations, which suggest that different cross-linked structures are formed throughout the phase segregated morphology. Published by Elsevier B.V.