Unexpectedly, the product analysis in microwave CH4/O-2 discharge by Li+ ion attachment mass spectrometry has brought forth a detection in the formation of the hydrogen peroxide H2O2. Furthermore, the observation of the minor H2O2H+ peak gives indirect evidence of H2O2 formation. The studies have been done in terms of discharge conditions with a view of increasing the product rate and to confirm the possible production of this compound in the gas phase.