Single-crystal electron spin resonance characterizations of several classes of ferric porphyrin complexes are proposed. The detailed description of electronic structure provided by these measurements is an essential step toward a mechanistic understanding of electron transfer in heme protein systems. Knowledge of the electronic configuration of these complexes over the full range of potential heme environments should allow accurate prediction of the changes in electron distribution that occur in response to changes in heme ligation. Iron porphyrin complexes unique in the variety of drastically different electronic structures adopted as a result of relatively minor changes in axial ligation. With stong axial ligands, ferric porphyrins adopt a low-spin (S=1/2) ground state. An initial series of investigations is designed to provide the basis for the prediction of spin density distribution and frontier orbital energies for low-spin ferric porphyrins with a wide range of axial ligands. Subsequent experiments will test the accuracy of these predictions. Another series of measurements involves a Jahn-Teller active low-spin ferric porphyrin complex that existsin the solid state as an equilibrium mixture of two vibronix forms. This complex provides an opportunity to explore the effect of porphyrin distortions on the electronic structure. Weaker axial ligation results in a high-spin (S=5/2) ground state; in the "crossover" region an equilibrium mixture of low-spin and high-spin species are observed. Electron spin resonance characterizations of several high-spin species are planned in conjunction with a multiple-temperature x-ray structural analysis of the spin equilibria observed for a class of sulfur ligated ferric porphyrin complexes. The absence of axial ligands results in an intermediate spin (S=3/2) ground state; in the "crossover" region between the high-spin and intermediate-spin limits one observes a single species with an "admixed" spin-state. Experiments are planned to explore the effect of axial ligand hydrogen bonding on the extent of "admixture." This effect represents another way that the hem produces drastic changes in electron distribution in response to changes in its environment.