Recent Mössbauer and EPR studies of two ferredoxins and of aconitase have given evidence for a three-iron cluster, probably of a [3Fe-3S] type. The studies of the oxidized EPR-active centers have shown that the three iron sites are characterized by significantly different magnetic hyperfine coupling constants. For the ferredoxin from Azotobacter vinelandii, for instance, we have observed A1 = -41 MHz, A2 = +18 MHz, and [A3] = 5 MHz. We demonstrate here that the magnetic properties of the clusters can be explained with a simple model of three high-spin ferric ions (S = 5/2) exchange-coupled to a system spin S = 1/2. The model assumes isotropic exchange and different couplings between the iron sites. The results show that the three sites have intrinsic hyperfine interactions similar to those of ferric rubredoxin; the differences in the observed interactions reflect the geometrical features of spin coupling. Furthermore, the three exchange coupling constants are equal within a factor of 2. This implies that the three-iron cluster is a single covalently linked structure and should not be considered as a [2Fe-2S] cluster weakly coupled to a third iron atom.