Heme oxygenase is the first and rate limiting enzyme of the microsomal heme degradation pathway. Heme (iron protoporphyrin-IX), a co-factor and substrate of the enzyme, is catabolized through a process in which the key step involves the hydroxylation of the alpha-meso carbon of the porphyrin macrocycle by heme-bound oxygen. To study the mechanism of this reaction, we have formed the metastable O-2 adduct of the heme-heme oxygenase complex and observed that its resonance Raman spectrum displays an oxygen isotope shift pattern unlike those of any other O-2-bound heme proteins. Analysis of the spectra suggests that the Fe-O-O unit is highly bent, showing that steric interactions between the bound O-2 and the residues of the distal pocket result in a unique mechanism of oxygen activation. We propose that the terminal oxygen atom is in van der Waals contact with an alpha-meso carbon of the porphyrin ring.