The structure of the recombinant medium chain alcohol dehydrogenase (ADH) from the hyperthermophilic archaeon Aeropyrum pernix has been solved by the multiple anomalous dispersion technique using the signal from the naturally occurring zinc ions. The enzyme is a tetramer with 222 point group symmetry. The ADH monomer is formed from a catalytic and a cofactor-binding domain, with the overall fold similar to previously solved ADH structures. The 1.62 A resolution A.pernix ADH structure is that of the holo form, with the cofactor NADH bound into the cleft between the two domains. The electron density found in the active site has been interpreted to be octanoic acid, which has been shown to be an inhibitor of the enzyme. This inhibitor is positioned with its carbonyl oxygen atom forming the fourth ligand of the catalytic zinc ion. The structural zinc ion of each monomer is present at only partial occupancy and in its absence a disulfide bond is formed. The enhanced thermal stability of the A.pernix ADH is thought to arise primarily from increased ionic and hydrophobic interactions on the subunit interfaces.