Assimilatory sulfite reductase contains a sirohaem that is very weakly coupled to a [4Fe-4S] cubane, i.e. five iron atoms in total. Dissimilatory sulfite reductase is a complex system with 20 Fe atoms/α2β2γ2 hexamer. A recent revision of the purification procedure for the Desulfovibrio vulgaris dissimilatory enzyme has afforded a preparation of only 10 Fe atoms hexamer, this has led to the conclusion that the topology of prosthetic groups parallels that of the assimilatory system [Wolfe, B. M., Lui, S. M. & Cowan, J. A. (1994) Eur. J. Biochem. 223, 79–89]. The new purification procedure has been reproduced but the claimed molecular properties are not reproducible. The highly purified, active desulfoviridin contains 20, not 10, Fe atoms/molecule; the sirohaem is extensively demetallated, not metallated; and the S= 9/2 iron-sulfur cluster is present, not absent.