Photolysis of the C-3 . H2O complex in an argon matrix with lambda greater than or equal to 400 nm leads to the formation of propynal and tricarbon oxide. One pathway to the formation of propynal has been shown experimentally and theoretically to involve the intermediate hydroxypropadienylidene (HPD). This intracomplex mechanism involves the photoinitiated insertion of C-3 into an OH bond of water followed by migration of H atoms along the CCCO backbone to form the end product, propynal. The mechanism by which tricarbon oxide, C3O, is formed is examined in this investigation. To probe this pathway, the H2C3O potential surface, including H atom loss and later attachment, has been calculated at the MP2/6-31G* level of theory. It is shown that two photoconversion pathways exist. One leads to HPD (and thence to propynal), while the other involves H atom loss to a HC3O intermediate, which then either loses another hydrogen atom to form C3O or captures a hydrogen atom to produce propynal. In addition, bimolecular mechanisms involving C atoms, C-2, and CnO molecules are explored to determine whether they might also yield C3O. Finally, the possible importance of all these reactions in the production of C3O and propynal in the mantles of interstellar dust grains is discussed.