All methanotrophic bacteria express a membrane-bound (particulate) methane mono-oxygenase (pMMO). In the present study, we have investigated pMMO in membrane fragments from Methylococcus capsulatus (strain M). pMMO contains a typical type-2 Cu2+ centre with the following EPR parameters: gz 2.24, gx,y 2.06, ACuz 19.0mT and ACux,y 1.0mT. Simulation of the Cu2+ spectrum yielded a best match by using four equivalent nitrogens (AN = 1.5mT, 42MHz). Incubation with ferricyanide neither changed nor increased the amount of EPR-active Cu2+, in contrast with other reports. The EPR visible copper seems not to be part of any cluster, as judged from the microwave power saturation behaviour. Continuous-wave electron—nuclear double resonance (CW ENDOR; 9.4GHz, 5–20K) experiments at g⊥ of the Cu(II) spectrum show a weak coupling to protons with an AH of 2.9MHz that corresponds to a distance of 3.8Å (1Å≡0.1nm), assuming that it is a purely dipolar coupling. Incubation in 2H2O leads to a significant decrease in these 1H-ENDOR intensities, showing that these protons are exchangeable. This result strongly suggests that the EPR visible copper site of pMMO is accessible to solvent, which was confirmed by the chelation of the Cu2+ by diethyldithiocarbamic acid. The 1H and 14N hyperfine coupling constants confirm a histidine ligation of the EPR visible copper site in pMMO. The hyperfine structure in the ENDOR or EPR spectra of pMMO is not influenced by the inhibitors azide, cyanide or ammonia, indicating that they do not bind to the EPR visible copper. We compared pMMO with the type-2 Cu2+ enzyme, dopamine β-mono-oxygenase (DβM). For DβM, it is assumed that the copper site is solvent-accessible. CW ENDOR shows similar weakly coupled and 2H2O-exchangeable protons (2.9MHz), as observed in pMMO, as well as the strongly coupled nitrogens (40MHz) from the co-ordinating N of the histidines in DβM. In conclusion, the resting EPR visible Cu in pMMO is not part of a trinuclear cluster, as has been suggested previously.