Two [NiFe] hydrogenases extracted from the hyperthermophilic bacterium Aquifex aeolicus (Aa) are studied at a pyrolytic graphite electrode using cyclic voltammetry. The two hydrogenases have been shown to exhibit very similar behavior. They are able to catalyze under a nitrogen or a hydrogen atmosphere, the direct hydrogen production from protons and the reverse reaction of hydrogen oxidation directly in the absence of any promoter. The effect of experimental parameters (enzyme concentration, pH, ionic strength) are investigated. The electrocatalytic activity has been shown to increase when the temperature is increased within the range 22-65 degrees C in agreement with the evolution of the enzymatic activity. Using electrochemical techniques concomitantly with quartz crystal microgravimetry, it is demonstrated that the electrocatalysis is largely governed by the strong adsorption of hydrogenases on the electrode surface. The similarity between the two Aa hydrogenases and the hydrogenase from another hyperthermophilic bacterium, Pyrococcus furiosus, is discussed. (c) 2004 Elsevier B.V. All rights reserved.