First-principles quantum chemical methods have been employed to simulate and predict the electronic effects of peripheral polyhalogenation in porphyrins. Hartree-Fock theory performs unexpectedly poorly in calculations of valence ionization potentials of polyhalogenated porphyrins. In contrast, the results of local density functional calculations of the lowest ionization potentials of free base porphyrin, meso-tetrafluoroporphyrin, and beta-octachloroporphyrin are consistent with existing electrochemical information. In the absence of macrocycle buckling due to steric interactions with other substituents, meso-tetrafluorination and beta-octachlorination increase the first ionization potential of free base porphyrin by less than or equal to 0.1 and similar to 0.6 eV, respectively.