(CO.H)2

from The Collaborative International Dictionary of English v.0.48

Glyoxal \Gly*ox"al\, n. [Glycol + oxalic + aldehyde.] (Chem.)
   A white, amorphous, deliquescent powder, {(CO.H)2}, obtained
   by the partial oxidation of glycol. It is a double aldehyde,
   between glycol and oxalic acid.
   [1913 Webster]