Electrochemical properties of substituted free base meso-tetraphenylporphyrins (H2T(o,o'-X)PP, H2T(o-X)PP, and H2T(p-X)PP, where X = OCH3, CH3, H, F, or Cl on the phenyl rings) are examined by cyclic voltammetry. When a substituent is located only at the para position of the meso-phenyl group, the difference between the first and second oxidation potentials (Delta E-ox, i.e., E-2(ox) - E-1(ox)), is generally significantly smaller than those of the H(2)TPPs with bulky o,o'-substituents on the phenyl group. This trend is elucidated with density functional theory calculations and attributed mainly to the sterically controlled pi-conjugation of the meso-phenyl groups to the central porphyrin ring, rather than the often discussed deformation of porphyrin.