Hydrogenases catalyze the oxidation and production of H(2). The fact that they could be used in biotechnological devices if they resisted inhibition by O(2) motivates the current research on their inactivation mechanism. Direct electrochemistry has been thoroughly used in this respect but often in a qualitative manner. We propose a new and precise chronoamperometric method for studying the anaerobic inactivation mechanism of hydrogenase, which we apply to the oxygen-tolerant NiFe enzyme from Aquifex aeolicus . We demonstrate that the voltammetric data cannot be used for measuring the reduction potential of the so-called NiB inactive state, even in the small scan rate limit. We show that the inactivation mechanism proposed for standard (oxygen-sensitive) NiFe hydrogenases does not apply in the case of the enzyme from A. aeolicus . In particular, the activation and inactivation reactions cannot follow the same reaction pathway.