Mass spectrometry based experiments show that CH2=O.+ is cleanly generated in the spontaneous decarbonylation of low energy glyoxal ions, O=CH-CH=O.+. Likewise, glyoxilic acid ions, O=CH-C(OH)=O.+, decarboxylate into CH2=O.+, but in admixture with some HCOH.+. A mechanistic analysis using the CBS-QB3 model chemistry shows that the above ions first rearrange into hydrogen bridged radical cations, HCO-H.+...C=O and HCO-H.+...O=C=O, respectively. Solitary ions HCOH.+ do not isomerize into CH2=O.+ but the CO or CO2 molecule in the above complexes catalyzes this transformation prior to dissociation. CO is the better catalyst because its proton affinity is closer to the criterion for efficient proton-transport catalysis. (C) 2004 Elsevier B.V. All rights reserved.