SINCE 1864, when Baeyer stated his formaldehyde hypothesis, numerous attempts have been made to obtain formaldehyde in vitro from carbon dioxide and water on exposure to light. Usher and Priestley,1 Baly, Heilbron and Barker,2 Dhar and co-workers,3 Mezzadroli and collaborators,4 and others, obtained evidence of formaldehyde formation from carbonic acid or bicarbonates in presence or absence of catalysts when exposed to light. On the other hand, Spoehr,5 Baur and Rebman,6 Potter and Ramsperger,7 Bell,8 Emerson,9 Zschiele,10 and Mackinney11 obtained negative results, although the last-named worker made the following statement: “The status of this problem is extraordinarily involved, though it can hardly be doubted that some workers have succeeded in obtaining formaldehyde in vitro”. Recently, Baly and co-workers12 seem to contradict their earlier results.