Resonance Raman spectroscopy has been used to observe changes in the iron–ligand stretching frequency in photo-product spectra of the proximal cavitymutant ofmyoglobin H93G. The measurements compare the deoxy ferrous state of the heme iron in H93G(L), where L is an exogenous imidazole ligand bound in the proximal cavity, to the pho-tolyzed intermediate of H93G(L)*CO at 8 ns.