The isotopic composition of dihydrogen generated from formaldehyde in highly basic solutions has been investigated. It is shown that two pathways are available for the reaction. In the first pathway, one hydrogen atom is aquated hydrogen and the other is methylene-hydrogen. In the second pathway both hydrogen atoms originate from the methylene moiety of the formaldehyde. When the source of dihydrogen is the corresponding reaction of glyoxylate, only the first pathway is observed. The changes of the Mulliken charges and the geometrical parameters in dependence on the number of H-bonded water molecules support the experimental finding that the probability of both hydrogen atoms originating from the methylene moiety of formaldehyde is higher. (C) 2004 Elsevier B.V. All rights reserved.