Abstract Proton as well as deuteron ENDOR (electron-nuclear double resonance) spectroscopy were performed of methanol dehydrogenase and pyrrolo-quinoline semiquinone (PQQH.). Samples were examined in H 2O- and 2H 2O-containing buffers at 4.2 °K with Ka-band (33.5 GHz) frequency. Measurements of the enzyme in 2H 2O revealed that the signals observed around the proton free-precession frequency belong to exchangeable protons. Therefore, our earlier assumption (R. de Beer et al. (1979) J. Chem. Phys. 70, 4491–4495) that these signals originate from protons in the aromatic ring of PQQH. is incorrect. The proton matrix signal of the enzyme in H 2O and 2H 2O are nearly similar, while a deuteron matrix signal is not observed in the latter case. It is concluded, therefore, that the coenzyme is situated in a hydrophobic site of the enzyme.