A systematic study of the properties and reactivities of iron(III) porphyrin peroxo complexes containing ligands with different electronic properties has been undertaken. While the spectral properties of the peroxo complexes do not show much variation, the stability of the complexes is dramatically increased by employing very electron-poor ligands such as 5,10,15,20-(pentafluorophenyl)porphyrin. The nucleophilicity of the peroxo moiety is decreased considerably by such electron-poor ligands : unlike all other iron(III) porphyrin peroxo complexes examined here, the perfluorinated peroxo complex does not epoxidize electron-poor olefins. However, this complex appears to have an unusually strong desire for a second axial ligand : it binds reversibly to triphenylphosphine. This interesting binding pattern is also observed for the perfluorinated iron-chloro complex. None of the complexes are capable of reacting with electron-rich olefins such as tetramethylethylene or with triphenylphosphine. Possible transition states for nucleophilic epoxidation are discussed.