In most cases, the crystallisation of a compound depends upon the material being pure and that it have a stable configuration or that the molecule will assume a reproducible configuration under the proper conditions. The task of the crystallographer is to get the molecule to crystallise while it is in the stable and reproducible condition. Unfortunately there are other proteins that are unstable due to the presence of extremely reactive or sensitive metal centres, cofactors or prosthetic groups and tend to become denatured easily. Success in obtaining crystals depends upon taking steps to obviate the instability before, during and after crystal growth. The generally unstable nature of the 2[4Fe-4S] centres in the bacterial ferredoxins causes these unusual redox proteins to be diffcult in producing and maintaining diffraction quality crystals. Conditions to induce relatively rapid crystallation coupled with techniques to keep the crystallisation experiments semi-anaerobic or anaerobic are described.