The effect of electron radiation on mono- and di-methyl pendant poly(p-phenylene benzobisthiazole) polymers (MePBZT and DiMePBZT, respectively) has been investigated. MePBZT is chemically stable (as measured by C-13 NMR) upon exposure to 1 Grad electron radiation at room temperature. The irradiation of DiMePBZT at 225degreesC was also carried out, which was found to have no effect up to a dosage of 850 Mrad. Thus, the additional molecular mobility brought about by heating (225degreesC) did not enhance the reactivity of DiMePBZT. The compressive and tensile properties of DiMePBZT fiber remained unchanged at lower radiation dosages. At a dosage of 850 Mrad, the tensile properties were found to decrease substantially, which may be attributed to defects observed in these fibers. The defects can be attributed to the effects of electron radiation, rather than prolonged heating at 225degreesC.