The bacterial chemotaxis protein of Desulfovibrio vulgaris DcrH (DcrH-Hr) functions as an O2-sensing protein. This protein has a hemerythrin-like domain that includes a nonheme diiron center analogous to the diiron center of the hemerythrin (Hr) family. Interestingly, the O2 affinity of DcrHHr is 3.3 × 106 M-1, a value 25-fold higher than that of the Pectinaria gouldii Hr. This high affinity arises from the fast association of the O2 ligand with DcrH-Hr (kon = 5.3 × 10 8 M-1 s-1), which is made possible by a hydrophobic tunnel that accelerates the passage of the O2 ligand to the diiron site. Furthermore, the autoxidation kinetics indicate that the rate of autoxidation of DcrH-Hr is 54-fold higher than that of P. gouldii Hr, indicating that the oxy form of DcrH-Hr is not stable toward autoxidation. More importantly, a mixed-valent state, semimetR, which was spectroscopically observed in previous Hr studies, was found to be stable for over 1 week and isolable in the case of DcrH-Hr. The high-resolution crystal structures of the semimetR- (1.8 Å) and met-DcrH-Hr (1.4 Å) indicate that the semimetR- and met- DcrH-Hr species have very similar coordination geometry at the diiron site.