[en] Gas-processing metalloenzymes are of interest to future bio- and bioinspired technologies. Of particular importance are hydrogenases and nitrogenases, which both produce molecular hydrogen (H${}_2$) from proton (H${}^{+}$) reduction. Herein, we report on the use of rotating ring-disk electrochemistry (RRDE) and mass spectrometry (MS) to follow the production of H${}_2$ and isotopes produced from deuteron (D${}^{+}$) reduction (HD and D${}_2$) 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 H${}_2$-producing metalloenzymes and the design of bioinspired catalysts for H${}_2$ production and N${}_2$ fixation. (© 2021 Wiley-VCH GmbH)