The hybrid cluster proteins from the sulfate reducing bacteria Desulfovibrio desulfuricans ATCC 27774 ( Dd) and Desulfovibrio vulgaris strain Hildenborough ( Dv) have been isolated and crystallized anaerobically. In each case, the protein has been reduced with dithionite and the crystal structure of the reduced form elucidated using X-ray synchrotron radiation techniques at 1.25 A and 1.55 A resolution for Dd and Dv, respectively. Although the overall structures of the proteins are unchanged upon reduction, there are significant changes at the hybrid cluster centres. These include significant movements in the position of the iron atom linked to the persulfide moiety in the oxidized as-isolated proteins and the sulfur atom of the persulfide itself. The nature of these changes is described and the implications with respect to the function of hybrid cluster proteins are discussed.