ESR spectra of oxochromium(V) porphyrins were obtained to investigate the electronic structure of the Cr-V=O moiety. At room temperature, the chlorooxochromium(V) meso-tetramesitylporphyrin complex 1-Cl exhibits an isotropic ESR signal, split into nine signals due to hyperfine coupling with four equivalent N-14, at g = 1.982. A small isotropic satellite signal, split into a quartet due to hyperfine coupling by the Cr-53 isotope is also observed. At 77 K, 1-Cl shows an anisotropic ESR signal with g(parallel to) = 1.986 and g(perpendicular to) = 1.980 and small satellite g(parallel to) signals arising from hyperfine coupling with the Cr-53 isotope. The ESR parameters of 1-Cl, obtained from ESR spectral simulation, show that the pi-bonding character of the Cr-V=O bond is not as strong as that of the Cr-V=N bond. To explore substituent effects on the bond character of the Cr-V=O moiety, ESR spectra of perchloratooxochromium(V) porphyrins having various electronegative substituents were recorded. With an increase in electronegativity of the meso-substituent, the gi value and the Cr-53 hyperfine coupling constant (/a(Cr-53)/) are increased. These changes suggest an increase in the ct-bonding character of the Cr-V=O moiety with an increase in the electronegativity of the meso-substituent. Furthermore, the hyperfine coupling constant of the oxo ligand, /a(O-17)/, obtained from ESR spectra of O-17-labeled oxochromium(V) porphyrin is increased with an increase in electronegativity of the meso-substituent. This indicates that the radical character of the oxo ligand is increased by the electronegative meso-substituent. These data suggest that the pi-antibonding orbital of Cr=O is destabilized with an increase in electronegativity of the meso-substituent, which may facilitate bond breaking of the Cr=O moiety in an epoxidation reaction.