X-ray crystallographic and ab initio molecular orbital analyses are presented for three model compounds of methyl-substituted poly(p-phenylenebenzobisthiazole), PBZT, where monomethyl and dimethyl substitutents are located on the phenylene moiety. The barrier to phenylene rotation, a factor considered to be important for an understanding of the mechanical, electronic, and nonlinear optical properties of PBZT and related rigid-rod heterocyclic polymers, is calculated for each compound. Ortho substitution with a monomethyl group substantially lowers the rotational barrier and profoundly changes the shape of the rotational potential, whereas meta substitution has only a negligible effect. Discrepancies between experimental and theoretical phenyl torsion angles are attributed to crystal packing forces. Ab initio results differ quantitatively from semiempirical molecular orbital findings. Good agreement is observed between crystallographic and computed bond lengths and angles.