The molecular structures of some intermediate and end products of the fc.rmose reac­tion are determined by means of coupled gas chromatography/mass spectrometry and by PFT 13C NMR spectroscopy. It is shown that glycolaldehyde, glyceraldehyde and dihydroxiacdone, which are intermediate products, are not single compounds but mixtures of monomers: hydroxicarbonyl com­pounds, epoxides, hydrates and dimers: half acetalrand full acetals. The structure of KUSIN's prod\lct, which is a key substance in LANGENBECK's theory ofautocatalysis is confirmed to be hydroximethyl­benzoine. Only a minor tendency of a fission into benzoine and "active formaldehyde" but decompo­sition of the catalyst by fission into benzoic acid is observed. The structure of UTKIN's product is confirmed to be a branched chain ketohexose caII~d dendroketose. Using heterogenous call;ium­hydroxide as a catalyst UTKIN's yields of dendroketose are doubled.