In order to influence the compressive strength of the rigid rod polymeric fibers, methyl pendent poly (p-phenylene benzobisthiazole) fibers have been heat treated in the 400 to 550 degrees C temperature range in air and in nitrogen for varying times to achieve intermolecular crosslinking. These fibers have been examined using Fourier transform infrared (FTIR) spectroscopy, C-13 solid-state nuclear magnetic resonance (NMR) swelling behavior, and scanning electron microscopy. C-13 NMR has also been carried out on solutions of as-spun fibers. Fibers heat-treated at 400 degrees C, both in nitrogen and in air, up to heat-treatment times of 60 min are insoluble in 99% chlorosulfonic acid, however no direct evidence of crosslinking has been obtained for these fibers using spectroscopic techniques, suggesting that in these fibers the degree of crosslinking must be very low. Evidence that methyl groups are precursors to certain crosslinks was first seen via a weak methylene resonance in C-13 solid-state NMR, corresponding to about 2% of the original methyl intensity, in a sample heat-treated at 450 degrees C in air. Fibers heat-treated in nitrogen at 550 degrees C for 10 minutes do not exhibit any swelling in chlorosulfonic acid, are brittle, have lost most methyl groups; however, some CH2 groups form. In this fiber, the carbon intensity for the CH2 group in the C-13 solid-state NMR is 18% of the intensity for the CH3 group in the as-spun fiber. The fibers heat-treated at 400 and 450 degrees C show a fibrillar morphology, while the fibrillar morphology is not observed in the fibers heat-treated at 550 degrees C in nitrogen for 10 min. Based on this work, it is our judgment that if heat treatment of this material is to improve compressive strength, the heat treatment protocol of time and temperature will probably be critical and the highest temperatures of exposure will probably lie in the 450 to 550 degrees C range.