H2O2+ ions are shown to be formed in ionized O-2/CH4 mixtures from the secondary reaction of CH2O+ with O-2. By high-resolution mass spectrometric techniques, the peroxide HOOH structure and the H2CO structure of ionized formaldehyde are assigned to the [H2O2](+) ions and the [CH2O](+) reactants, respectively. The [CH4O2](+) intermediate of the reaction between O-2(+) and CH4 is characterized as a mixture of the loosely bound [CH4 center dot O-2](+) Complex and the theoretically predicted insertion product CH3OOH+. The CH3OOH+ intermediate, precursor of the most abundant ionic product CH2OOH+, has been characterized for the first time as a long-lived species. (c) 2007 Elsevier B.V. All rights reserved.