Possibilities for the titrimetric determination of dimeric glycolaldehyde (m.p. 94–96° C) anddl-glyceraldehyde (m.p. 138–141° C) have been investigated. The systematic errors based on different functional groups depend on the stability of the cyclic semiacetals and on the subsequent equilibria established after their depolimerisation. The semiacetal ofdl-glyceraldehyde is more stable than that of glycolaldehyde. 95–96% and 98–99% of the real content were found by the hydrogensulphite and hydroxylamine methods, respectively. Glycolaldehyde decomposes rather quickly in aqueous solution and the equilibrium can be shifted towards the monomeric form by both the aldehyde reactions. The hydroxylamine method proved to be more advantageous for the determination of the dimeric hydroxyaldehydes, which cristallise in the form of cyclic se