Resonance Raman spectroscopy has been applied to two distinct temporal species of indoleamine 2,3-dioxygenase during catalytic turnover. We have identified two oxygen-isotopesensitive Raman modes at 569 and 798 cm(-1) for the two respective species. The (OO)-O-16-O-18 analysis of the 798 cm(-1) band indicates the existence of a ferryl-oxo herne, which is inconsistent with the previously proposed reaction mechanism. The present study thus provides a physical basis for the structures of the possible reaction intermediates.