The reasons that the temperature dependence of the NMR isotropic shifts of model ferrihemes and ferriheme proteins deviate from Curie behavior have been analyzed by considering the energies of the valence orbitals of the metal and the porphyrinate. For low-spin Fe(III), overlap of the e-symmetry pi orbitals of a symmetrical porphyrin ring and the d(pi) orbitals of the metal produces two low-energy molecular orbitals that are mainly porphyrin in character and are filled and two high-energy (valence) molecular orbitals that are mainly metal in character and contain three electrons. The odd electron in the valence set thus gives rise to the spin delocalization that results in the observed contact shift pattern of these systems. Unsymmetrical substitution and/or presence of a planar axial ligand that is prevented from rotation removes the degeneracy of these e(pi) orbitals, producing a system in which the energy separation between the two formerly degenerate pi orbitals, Delta E(pi), is of the order of only tens of cm(-1) for the former or quite large (several times k(B)T) for the latter. In either case, both orbitals are utilized for spin delocalization to a significant extent as the temperature is varied, according to their varying Boltzmann populations. Such a two-level system obeys a modified Curie law that takes into account the thermal population of the two levels as a function of temperature. In fact, the temperature dependence of some of the contact shifts of model hemes or heme proteins may show anti-Curie behavior if Delta E(pi) is large compared to k(B)T at ambient temperatures.