Abstract The conditions of the co-existence of hydrogen dioxide and formaldehyde, and the nature of their interaction, are of special interest in connection with the theories which have been advanced to account for the photosynthesis of carbohydrates in plants. As the initial stage in the assimilation of carbon dioxide, Bach suggested that carbon dioxide in presence of water gives rise to percarbonic acid (H2CO4) and formaldehyde, whereas Usher and Priestley consider that the primary products are hydrogen dioxide for formaldehyde, the change being reversible in the sense CO2 + 3H2O ⇌ H.CHO + 2H2O2. This view has been opposed by various authors, one ground for objection being the, presumably, incompatible nature of the two compounds named.