In this report we examine the steady-state optical absorption, steady state and transient vibrational structure, and ligand rebinding kinetics of (CO)Fe-II protoporphyrin IX ((CO)Fe(II)PPIX) in dimethyl sulfoxide (DMSO). Steady state optical absorption and resonance Raman spectra of this complex are characteristic of heme iron that is six-coordinate and low-spin. Absorption maxima are observed at 415 nm (Soret), 568 nm (alpha-band), and 535 nm (beta-band), and vibrational bands are observed at 1370 cm(-1) (nu(4)), 1496 cm(-1) (nu(3)), 1551 cm(-1) (nu(38)), 1584 cm(-1) (nu(2)), and 1626 cm(-1) (nu(C=C)). The transient absorption difference spectrum subsequent to photolysis displays an absorption maximum at 433 nm and a minimum at 414 nm that decays biphasically with pseudo-first-order rate constants of (2.11 +/- 0.024) x 10(6) s(-1) and (2.29 +/- 0.036) x 10(2) s(-1). The corresponding transient resonance Raman spectrum displays vibrational bands at 1356 cm(-1) (nu(4)), 1470 cm(-1) (nu(3)), 1559 cm(-1) (nu(2)), 1584 cm(-1) (nu(37)), and 1618 cm(-1) (nu(C=C)) These results are consistent with the formation of a five-coordinate and high-spin transient species that is identical to the photolytic transient observed upon photolysis of the (DMSO)(2)Fe(II)PPIX complex (i.e., (DMSO)Fe(II)PPIX). The decay of this transient species is consistent with the binding of a DMSO to the five-coordinate heme iron followed by substitution of a bound DMSO with CO.