The mechanism of reaction of Fe Fe hydrogenases with oxygen has been debated. It is complex, apparently very dependent on the details of the protein structure, and difficult to study using conventional kinetic techniques. Here we build on our recent work on the anaerobic inactivation of the enzyme [Fourmond et al. Nat. Chem. 2014, 4, 336-342] to propose and apply a new method for studying this reaction. Using electrochemical measurements of the turnover rate of hydrogenase, we could resolve the first steps of the inhibition reaction and accurately determine their rates. We show that the two most studied FeFe hydrogenases, from Chlamydomonas reinhardtii and Clostridium acetobutylicum, react with O-2 according to the same mechanism, despite the fact that the former is much more O-2 sensitive than the latter. Unlike often assumed, both enzymes are reversibly inhibited by a short exposure to O-2. This will have to be considered to elucidate the mechanism of inhibition; before any prediction can be made regarding which mutations will improve oxygen resistance. We hope that the approach described herein will prove useful in this respect.