The unsolvated porphyrin compound (trifluoromethanesulfonato)(meso-tetraphenylporphinato)iron(III), [FeIII(TPP)(OSO2CF3)], has been structurally characterized by single-crystal X-ray diffraction in a monoclinic phase at 298 K and in a triclinic phase at 293 K and 188 K. While only one type of molecular site is found in the monoclinic phase, the temperature-dependent structural parameters and magnetic susceptibility data (5.82 mu(B) at 293 K; 4.86 mu(B) at 20 K) together indicate the existence of two crystallographically and magnetically distinct crystal lattice sites in the triclinic phase. One site of the triclinic phase (molecule 1) is unique in that its structure is temperature dependent, whereas the second spin-admixed site (molecule 2) has a structure which is essentially independent of temperature. This distinct site assignment has been further investigated by low-temperature Mossbauer (6, 77, 150 K) and EPR (11 K) spectroscopy which suggest different spin ground states for molecules 1 and 2. As such, [FeIII(TPP)(OSO2CF3)] represents the first report of spin-state isomerism in the solid state for a pentacoordinate [Fe(III)(porphinato)X] species possessing two crystallographically, magnetically, and spectroscopically distinct sites in the same unit cell. This triclinic phase is also the first report of molecule pairs of the same porphyrin complex interacting in very different ways in the same crystal lattice, in that molecule 1 pairs form pi-pi dimers whereas molecule 2 pairs do not. Crystal data for [FeIII(TPP)(OSO2CF3)] in the monoclinic phase at 293 K : space group P2(1)/a (No. 14), Z = 4, a = 14.249(4) angstrom, b = 17.630(4) angstrom, c = 15.175;5) angstrom, beta = 97.35(3)degrees. Crystal data for [FeIII(TPP)(OSO2CF3)] in the triclinic phase : space group P1BAR (No. 2), Z = 4; a = 13.709(3) angstrom, b = 26.230(3) angstrom, c = 11.782(2) angstrom, alpha = 97.81(1)degrees, beta = 114.84(2)degrees, and gamma = 89.04(2)degrees at 293 K; a = 13.474(8) angstrom, b = 25.939(9) angstrom, c = 11.666(9) angstrom, alpha = 97.91(5)degrees, beta = 114.28(5)degrees, and gamma = 88.98(4)degrees at 188 K.