The isocitrate dehydrogenases from the extremely halophilic Archaeon, Haloferax volcanii, and from the hyperthermophilic Archaeon, Sulfolobus solfataricus, have been purified to electrophoretic homogeneity. The purified enzymes have been characterised with respect to their cofactor specificities, subunit compositions and their salt and thermal stabilities. N-terminal amino acid sequences have been determined for both enzymes, and multiple alignments with sequences of bacterial and eukaryotic isocitrate dehydrogenases show that the archaeal enzymes most closely resemble the NADP-linked dimeric isocitrate dehydrogenases from the Bacteria.