Nitric oxide reductases (NOR) are heme-containing enzymes, which are involved in fungal or bacterial denitrification, and reduce nitric oxide (NO) to nitrous oxide (N₂O) fro detoxification. Fungal NOR is a water-soluble and cytochrome P450 type enzyme, while bacterial NOR is a membrane-integrated enzyme containing a heme/non-heme iron binuclear center as its active site. In both enyzmes, short-lived intermediates are appeared during the NO reduction reaction. For characterizing the coordination and electronic structures of the intermediates, we applied time-resolved IR spectroscopic and crystallographic techniques. The information obtained from these experiments will help us to understand the molecular mechanism of the NO reduction catalyzed by two different NOR enzymes.