Previous studies have shown that the solitary ketene-water ion CH 2 C( O)OH 2•+ (1) does not isomerize into CH 2 C(OH) 2 •+ (2), its more stable hydrogen shift isomer. Tandem mass spectrometry based collision experiments reveal that this isomerization does take place in the CH 2 O loss from low-energy 1,3-dihydroxyacetone ions (HOCH 2 ) 2 C O•+ . A mechanistic analysis using the CBS-QB3 model chemistry shows that such molecular ions rearrange into hydrogen-bridged radical cations [CH 2
C( O)O(H)-H· · ·OCH 2 ]•+ in which the CH 2 O molecule catalyzes the transformation 1 → 2 prior to dissociation. The barrier for the unassisted reaction, 29 kcal mol −1 , is reduced to a mere 0.6 kcal mol −1 for the catalysed transformation. Formaldehyde is an efficient catalyst because its proton affinity meets the criterion for facile proton-transport catalysis.