The conformation of substituted poly(p-phenylene terephthalate)s has been studied with molecular mechanics and Metropolis Monte Carlo methods. It was shown that substitution lowers the trans-gauche energy difference of the most important dihedral angle of the backbone more when the substituent is attached to the hydroquinone residue (HQ) than when it is attached to the terephthalic acid residue (TA). The substitution also lowers the trans-gauche energy barriers for both HQ and TA, but the gauche states are more stable in HQ. The trans region is wider for HQ compared with the unsubstituted monomer and TA. The mean squared radius of gyration, the mean squared end-to-end distance and the persistence length were calculated with the Metropolis Monte Carlo method for polymers with octyloxy side chains, when the side chain was attached either to the hydroquinone unit (PHQ8) or to the terephthalic acid unit (PTA8). In PHQ8 the side chains are oriented mainly parallel to the backbone; in PTA8 they are oriented more randomly. The main chain of PTA8 is more extended than that of PHQ8, owing to the wider trans minimum in PHQ8 than in PTA8.