The powder and single-crystal EPR spectra of Co-II(OEP) (OEP is the dianion of octaethylporphyrin) doped into a range of diamagnetic crystals including simple four-coordinate hosts, H-2(OEP), the triclinic B form of Ni-II(OEP), the tetragonal form of Ni-II(OEP) and Zn-II(OEP); five-coordinate hosts, (mu-dioxane)-{Zn-II(OEP)}(2) and (py)Zn-II(OEP); six-coordinate hosts, (py)(2)Zn-II(OEP) and (py)(2)Mg-II(OEP); and hosts containing fullerenes, C-60.2Zn(II)(OEP).CHCl3, C-70.Ni-II(OEP).C6H6.CHCl3, and C-60.Ni-II(OEP).2C(6)H(6) have been obtained and analyzed. Spectra were simulated using a program that employed the exact diagonalization of the 16 x 16 complex spin Hamiltonian matrix. The EPR spectra of these doped samples are very sensitive to the environment within each crystal with the crystallographic site symmetry determining whether axial or rhombic resonance patterns are observed. For Co-II(OEP) doped into tetragonal Ni-II(OEP) (which displays a very large g(perpendicular to) of 3.405 and a very small g(parallel to) of 1.544) and several other crystals containing four-coordinate metal sites, the g components could not be fit using existing theory with the assumption of the usual z(2) ground state. However, reasonable agreement of the observed EPR parameters could be obtained by assuming that the unpaired electron resides in an xy orbital in the four-coordinate complexes.