The crystal structures of three porphyrin diacid species, [H4OEP](ClO4)(2), [H(4)Tpp](ClO4)(2), and [H4TMP]-(ClO4)(2), have been determined from low-temperature X-ray diffraction data to delineate how the peripheral substituents of the porphyrin affect the overall molecular flexibility. [H4OEP](ClO4)(2) (\Cb\ = 0.67 Angstrom), [H4TMP](ClO4)(2) (\Cb\ = molecule 1), and [H4TPP](ClO4)(2) (\Cb\ = 0.93 Angstrom) show increasingly saddled core conformations with effective D-2d symmetry. The mean porphyrin-aryl group dihedral angles in [H4TPP](ClO4)(2) and [H4TMP](ClO4)(2) (molecule 1) are 27(2)degrees and 63(13)degrees, respectively. The steric bulk of the mesityl substituents in [H4TMP](2+) limits the range of observed porphyrin-aryl group dihedral angles to >50 degrees and, consequently, the magnitude of the core distortion. [H4TMP](2+) is therefore less flexible than [H4TPP](2+). Molecular mechanics calculations, using a modified version of MM2(87) and a newly developed force field for porphyrin diacids, correctly predict that the flexibility of mesotetraaryl porphyrin diacids decreases as the steric bulk of the peripheral substituents increases: [H4TPyP](2+) approximate to [H4TPP](2+) > [H4T-2,6-(OH)(2)PP](2+) approximate to [H4T-2,6-F2PP](2+) > [H4T-2,6-Cl2PP](2+) approximate to [H4TMP](2+). Grid searches of conformational space for [H(4)porphine](2+), [H4OEP](2+), [H4TPP](2+), and [H4T-2,6-Cl2PP](2+) located pairs of inversion-related minima with D-2d -saddled and C-2h,-stepped core conformations. The in vacuo strain energy barrier to inversion of the lowest-energy D-2d-saddled conformation increases from 0.45 kcal/mol in [H(4)porphine](2+) to 1.9 kcal/mol in [H4T-2,6-Cl2PP](2+). The calculations indicate that the relative stability and magnitude of distortion of the D-2d isomer increases as the steric bulk of the peripheral substituents increases; [H4OEP](2+) is therefore calculated to be less distorted than [H4TPP](2+), in agreement with the X-ray structures of these species.