Heme a, the iron-containing prosthetic group of the respiratory protein, cytochrome oxidase, has unusual substituents at two pyrrole carbons: a formyl at position z and a hydroxyfarnesylethyl at position 2 [l]. As a consequence, the optical properties of heme a are significantly different from those of protoheme [2]. In vivo, two hemes a occur per protein molecule. One is associated with cytochrome a and is low-spin, while the second is identified with cytochrome a3 and is high-spin [3]. The protein optical properties are thus a convolution of those of one low-spin and one high-spin heme a. The possible modulation of these basic features by protein-mediated interaction effects has rendered the absorption spectrum of cytochrome oxidase controversial [4,5] and several suggestions, ranging from an independent chromophore model [4,6] to an a:a3 exciton interaction model [7], have been made. The possibility that the longest wavelength visible transition in low-spin heme a compounds has porphyrin rr to formyl carbonyl n* charge transfer character has also been raised [8]. The latter hypothesis is of particular interest in light of current models for the role of the formyl group in enzyme function [9]. Resonance Raman spectroscopy is able to provide insight into the electronic properties of heme a in solution and in situ [ 10-121. In the experiments reported here we have investigated the Raman spectra of low-spin heme a complexes obtained with laser excitation in resonance with the 590 nm optical band. Our data demonstrate that this band corresponds to the heme a Qoo transition; no evidence of carbonyl involvement was found. The visible spectrum of reduced cytochrome oxidase is discussed with reference to the unusually high cr band oscillator strength of low-spin heme a complexes. A comparison of the low-spin model compound Raman data we have obtained with analogous data reported for the protein [ 131 shows a close correspondence between the two. We also note that the depolarization ratios for heme a vibrations enhanced by (Y band excitation are much lower than observed for protoheme compounds.