Monomethyl-pendant poly(p-phenylene benzobisimidazole) (MePBI) has been spun into fiber and compared with an analogous methyl pendant poly(p-phenylene benzobisthiazole) (MePBZT). From FTIR and WAXD, the former system has been found to exhibit intermolecular hydrogen bonding, while the latter possesses only weak van der Waals interactions. The thermomechanical properties of both systems, as examined by TGA, TMA, and DMS, are presented and discussed. A crystal structure of MePBI is proposed, which suggests intermolecular hydrogen bonding only between pairs of chains within the MePBI crystal. As compared to MePBZT and other rigid-rod polymers where the intermolecular hydrogen bond is not present, MePBI has higher compressive strength. The relatively high compressive strength of MePBI is attributed to increased intermolecular association, as opposed to differences in morphology.