In order to study the bonding of sulfoxides to iron(II) porphyrinates, an equilibrium study of Fe(TPP) with tetramethylenesulfoxide (TMSO) has been performed. UV-vis spectra at different concentrations of TMSO have shown distinct character belonging to three species: four-coordinate Fe(II)(TPP), five-coordinate [Fe(II)(TPP)(TMSO)], and six-coordinate [Fe(II)(TPP)(TMSO)(2)]. The isosbestic points for the low TMSO concentrations suggest that the equilibrium constant K, is much larger than K-2. Analysis of spectral data by the nonlinear least-squares program SQUAD gives K-1 = 267 and K-2 approximate to 1. Even though the five-coordinate species is the dominant species under the synthetic conditions, only the six-coordinate species was crystallized and characterized by an X-ray diffraction study. [Fe(TPP)(TMSO)(2)] (C52H44Fe-N4O2S2): monoclinic, P2(1)/c, a = 11.2580(3) angstrom, b = 15.9262(5) angstrom, c = 12.3930(4) angstrom, beta = 116.246(1)degrees, V= 1992.95(10) angstrom(3), Z= 2. X-ray crystallography demonstrates the complex is a low-spin bis-TMSO ligated species, The average Fe-N-p distance is 1.999(4) angstrom. The most important feature is that TOO is coordinated to iron(II) by the sulfur donors, not oxygen. The Fe-S distance is 2.2220(3) angstrom.