the simplest dialdehyde, OHC—CHO. In the crystalline and liquid states it is yellow; the vapor is green. Melting point, 15° C; boiling point, 50.4° C; density, 1.4 g/cm³ (20° C); index of refraction n_D^20.5 1.3826.

Glyoxal is prepared by dehydrogenation of ethylene glycol, HOCH₂—CH₂OH, over a copper catalyst or by treating tetrachloroethane, Cl₂CH—CHCl₂, with oleum, followed by hydrolysis of the resultant glyoxal sulfate,

Glyoxal is readily soluble in water, alcohol, and ether; it polymerizes readily during storage, particularly in the presence of traces of water. Polymeric glyoxal can be de-polymerized to the monomeric form by distilling over phosphorus pentoxide. In the presence of alkali it undergoes the intramolecular Cannizzaro reaction, yielding glycolic acid. Treatment with ammonia gives imidazole, and with aromatic o-diamines it gives pyrazine derivatives.