Archaeal dehydrogenases are often found to be of a specific class of dehydrogenase which has low sequence identity to the equivalent bacterial and eukaryotic counterparts. This paper focuses on two different types of hyperthermophilic dehydrogenase enzyme that have been cloned and over-expressed in Escherichia coli. The crystallographic structures of the apo form of GAPDH (glyceraldehyde-3-phosphate dehydrogenase) from Sulfolobus solfataricus and the related holo form of GAPDH from Methanothermus fervidus have been solved to high resolution. The zinc-containing structure of ADH (alcohol dehydrogenase) from Aeropyrum pernix has also been solved as a quaternary complex with the cofactor NADH and the inhibitor octanoic acid. The results show that despite the low sequence identity to the related enzymes found in other organisms the fold of the protein chain is similar. The archaeal GAPDH enzymes show a relocation of the active site which is a feature of evolutionary interest. The high thermostability of these three archaeal dehydrogenases can be attributed to a combination of factors including an increase in the number of salt bridges and hydrophobic interactions, a higher percentage of secondary structure and the presence of disulphide bonds.