Unimolecular dissociation of glyoxal via a three-body fragmentation channel has been studied by direct classical trajectory calculations using Hartree-Fock (HF) and hybrid density functional methods (BH&HLYP, B3LYP) with split valence and polarized basis sets [HF/3-21G, BH&HLYP/6-311G(d,p) and B3LYP/6-311G(d,p)]. The transition state for C2H2O2-->H-2 + 2 CO has a dihedral angle of 90-110 degrees between the carbonyl groups and a calculated barrier of similar to 59 kcal/mol above the trans conformer. To simulate the experimental conditions, trajectories were started from a microcanonical ensemble at the transition state with 4, 8, and 16 kcal/mol excess energy distributed among the vibrational modes and the transition vector. In agreement with experiment, the CO rotational distribution is very broad with a high [J]. However, the calculations yielded more CO vibrational excitation for the triple dissociation channel than observed for all channels combined. Hydrogen is produced with low J but significant vibrational excitation, in accord with experiment. Similar to trajectory studies on H2CO-->H-2 + CO, there is a good correlation between the energy released along the part of the reaction path where most of the H-2 bond length change occurs and the average vibrational excitation of the H-2 products.