X-ray crystal structures are reported for two sterically crowded porphyrins : 3,5,7,13,15,17-hexaethyl-2,8,12,18-tetramethylporphyrin (6, H2DAP) and (3,5,7,13,15,17-hexaethyl-2,8,12,18-tetramethylporphinato)nickel?? (II) (Ni6, NiDAP). In the NiDAP structure, steric crowding is relieved by the usual mechanism of the porphyrin adopting a nonplanar conformation. The crystal structure of H2DAP is unusual, as it shows an essentially planar conformation of the porphyrin macrocycle. In this case, steric crowding of the meso and pyrrole ethyl groups is relieved by a novel elongation of the macrocycle along the 5,15 axis. A detailed study is made of the solution conformations of H2DAP, NiDAP, and the corresponding zinc(II) complex (ZnDAP), using optical spectroscopy, variable-temperature proton NMR spectroscopy, and molecular mechanics calculations. It is suggested that the two species observed in low-temperature proton NMR studies of H2DAP and ZnDAP correspond to cis and trans conformations of the methyl groups of the meso ethyls, and not the syn and anti structures proposed previously (Maruyama; et al. J. Phys. Org. Chem. 1988, 1, 63). Crystal data : H2DAP (6), C36H46N4, M(r) = 534.8, monoclinic, P2(1)/c, a = 6.458(4) angstrom, b = 16.58(2) angstrom, c = 13.778(7) angstrom, beta = 99.78(4)-degrees, V = 1,454(2) angstrom3, Z = 2, D(x) = 1.222 Mg.m-3, lambda(Mo Kalpha) = 0.710 73 angstrom, mu = 0.072 mm-1, 130 K, R = 0.069 for 1769 reflections with F > 4.0sigma(F); NiDAP (Ni6), C36H44N4Ni, M(r) = 591.5, monoclinic, P2(1)/c, a = 14.350(8) angstrom, b = 16.043(6) angstrom, c = 15.049(6) angstrom, beta = 117.12(3)-degrees, V = 3,083(2) angstrom3, Z = 4, D(x) = 1.274 Mg.m-3, lambda(Mo Kalpha) = 0.710 73 angstrom, mu = 0.660 mm-1, 130 K, R = 0.072 for 3717 reflections with F > 4.0sigma(F).