The electronic and redox properties of the iron and tungsten centers in the aldehyde ferredoxin oxidoreductases (AORs) from Pyrococcus furiosus (Pf) and Pyrococcus strain ES-4 (ES-4) have been investigated by the combination of EPR and variable-temperature magnetic circular dichroism (VTMCD) spectroscopies. Parallel- and perpendicular-mode EPR studies of ES-4 AOR reveal a redox inactive "g = 16" resonance from an integer spin paramagnet. On the basis of the X-ray crystal structure of Pf AOR (Chan,M. K.; Mukund, S.; Kletzin, A.; Adams, M. W. W.; Rees, D. C. Science 1995, 267, 1463-1469), this resonance is attributed to a mononuclear high-spin Fe(II) ion at the subunit interface, although the possibility that this center is a carboxylate-bridged reduced diiron center in ES-4 AOR is also considered. Both enzymes have a [4Fe-4S](2+,+) cluster with unique electronic properties compared to known synthetic or biological [4Fe-4S](+) clusters, i.e. pure S = 3/2 ground state with g = 4.7, 3.4, 1.9 (E/D = 0.12 and D = +4 cm(-1)).