The gas-phase electron affinities (EAs) of a series of halogenated iron tetraphenyl porphyrins determined using ion cyclotron resonance mass spectrometry are reported. The EAs of the Fe(II) species vary approximately linearly with their condensed-phase reduction potentials. The EAs of the Fe(III) species with axial Cl ligands vary approximately linearly with condensed-phase reduction potentials. The lines for the Fe(III) and Fe(II) data sets are quite different, however, suggesting substantial solvent effects on the condensed-phase reduction process. Analysis in terms of the Born equation suggests a smaller effective size for the two Fe(III) species with the lowest degree of halogenation, which suggests in turn that those species accept an electron into a metal-based orbital while the remaining species accept an electron into a ligand-based orbital. The available catalytic activities of the various metalloporphyrins for the oxidation of isobutane to tert-butyl alcohol are covariant with the electron affinities. This covariance is discussed in the context of a catalytic mechanism, and it is suggested that the gas-phase electron affinity is a useful measure of electronic effects on catalyst activity.