The hyperthermophilic archaeon Pyrococcus furiosus contains a 4-Fe ferredoxin (Pf-Fd) that differs from most other 4Fe-Fd’s in that its [Fe4S4] cluster is anchored to protein by only three cysteinyl residues. Pf-Fd also is of interest because in its reduced form, [Fe4S4](+) the cluster exhibits both S = 1/2 and S = 3/2 spin states. Addition of excess cyanide ion converts the cluster exclusively to an S = 1/2 state (gl = 2.09, g(2) = 1.95, gs = 1.92); however, dialysis restores the EPR signal of native reduced protein, indicating that the cluster is not irreversibly altered by cyanide. Both the native protein and protein in the presence of excess cyanide ion (Pf-Fd-CN) were investigated here using the techniques of electron paramagnetic resonance (EPR) and electron-nuclear double-resonance (ENDOR) spectroscopy. No evidence for a strongly coupled solvent-derived hydrogen (H-1 or H-2) from an OH- or H2O ligand in either spin state of the [Fe4S4](+) cluster was observed, contrary to an earlier report. Rather, H-1,H-2 ENDOR characteristic of 4Fe-Fd’s was seen for both native Pf-Fd and Pf-Fd-CN. Pf-Fd-CN was further investigated using (CN-)-C-13 and (CN-)-N-15 ligands. C-13 and N-15 ENDOR indicated that a single cyanide ion bound directly, with the cluster showing an unusually small contact interaction (a(iso)(C-13) similar to -3 MHz, a(iso)(N-15) similar to 0). This is in contrast to cyanide bound to monomeric low-spin Fe(III)-containing proteins such as transferrin and myoglobin, for which the C-13 hyperfine coupling has a large isotropic component (a(iso)(C-13) approximate to -30 MHz).