Nickel(II) complexes of the geochemically significant four regioisomers of tetrahydrobenzoetioporphyrin, NiTHBP-A -B, -C, and -D, which contain a reduced benzo unit fused onto the C-beta atoms of a pyrrole ring, have been synthesized and structurally characterized by resonance Raman (RR) spectroscopy with variable-wavelength excitation. Spectra were obtained from CS2 and CH2Cl2 solution samples at room temperature excited at 406.7, 530.9, and 568.2 nm, in resonance with the porphyrin Soret, Q(1), and Q(0) electronic transitions, respectively. For comparison, similar RR spectra were measured for Ni(II) tetra-beta,beta-butanoporphyrin (NiTBuP) in which each pyrrole bears a tetrahydrobenzo exocyclic ring. These three excitation wavelengths, which selectively enhanced vibrational modes of different symmetry, the measured Raman band depolarization ratios, and the already available normal coordinates of Ni(II) octaethylporphyrin (NiOEP) permitted assignment of RR bands to nearly all of the porphyrin in-plane skeletal modes. The frequencies of skeletal modes above 1300 cm(-1) indicate more planar structures in solution for NiTHBP’s and NiTBuP relative to NIOEP and Ni(II) etioporphyrins. Several unique marker bands are also found for modes of the tetrahydrobenzo exocyclic rings, especially in the 568.2-nm excited spectra. The relative positions of the methyl and ethyl substituents have a marked influenced on the low- (350-550 cm(-1)) and mid-frequency (750-1100 cm(-1)) vibrational spectra, allowing the four NiTHBP regioisomers to be readily distinguished.