Gas‐processing metalloenzymes are of interest to future bio‐ and bioinspired technologies. Of particular importance are hydrogenases and nitrogenases, which both produce molecular hydrogen (H2) from proton (H+) reduction. Here, we report on the use of rotating ring disk electrochemistry (RRDE) and mass spectrometry (MS) to follow the production of H2 and isotopes produced from deuteron (D+) reduction (HD and D2) using the [FeFe]‐hydrogenase from Clostridium pasteurianum, a model hydrogen‐evolving metalloenzyme. This facilitates enzymology studies independent of non‐innocent chemical reductants. We anticipate that these approaches will be of value in resolving the catalytic mechanisms of H2 ‐producing metalloenzymes and the design of bioinspired catalysts for H2 production and N2 fixation.