Spinach ferredoxin-sulfite reductase (SiR) contains one siroheme and one Fe4S4 center per polypeptide subunit. The heme is entirely in the high-spin Fe3+ state in the oxidized enzyme. When SiR is photochemically reduced with ethylenediaminetetraacetate (EDTA)-deazaflavin, the free enzyme and its CN- and CO complexes show changes in absorption spectra associated with the heme even after the heme has been reduced from the Fe3+ to the Fe2+ state. With CO- or CN--SiR, these spectral changes are associated with the appearance of a classical "g = 1.94" type of EPR spectrum characteristic of reduced Fe4S4 centers. The line shapes and exact g values of the g = 1.94 EPR spectra vary with the nature of the ligand bound to the heme Fe. Photoreduction of free SiR results in production of a novel type of EPR signal, with g = 2.48, 2.34, and 2.08 in the fully reduced enzyme; this signal accounts for 0.6 spin per heme. (A small g = 1.94 type EPR signal, representing 0.2 spin per heme, is also found.) These data suggest the presence of a strong magnetic interaction between the siroheme and Fe4S4 centers in spinach SiR, this interaction giving rise to different EPR signals depending on the spin state of the heme Fe in the reduced enzyme.