The bacterial respiratory nitric-oxide reductase (NOR) catalyzes the respiratory detoxification of nitric oxide in bacteria and Archaea. It is a member of the well known super-family of heme-copper oxidases but has a [heme Fe–non-heme Fe] active site rather than the [heme Fe–CuB] active site normally associated with oxygen reduction. Paracoccus denitrificans NOR is spectrally characterized by a ligand-to-metal charge transfer absorption band at 595 nm, which arises from the high spin ferric heme iron of a μ-oxo-bridged [heme Fe(III)–O–Fe(III)] active site. On reduction of the nonheme iron, the μ-oxo bridge is broken, and the ferric heme iron is hydroxylated or hydrated, depending on the pH. At present, the catalytic cycle of NOR is a matter of much debate, and it is not known to which redox state(s) of the enzyme nitric oxide can bind. This study has used cyanide to probe the nature of the active site in a number of different redox states. Our observations suggest that the μ-oxo-bridged [heme Fe(III)–O–Fe(III)] active site represents a closed or resting state of NOR that can be opened by reduction of the non-heme iron.