The crystal and molecular structure of racemic bis(ethane-1,2- diamine)(thiooxalato-O,S)cobalt chloride hydrate ([Co(en)₂(mtox)] Cl,H₂O) has been determined by single-crystal X-ray diffraction techniques; all atoms, including hydrogens, have been refined by full-matrix least-squares procedures to a final R index of 0.032. The crystals are monoclinic, space group P2₁/c with a 7.548(1), b 14.249(2), c 12.317(2) Ǻ, β 109.27(1)°. The structure consists of discrete cations linked by hydrogen bonds to the chloride anion, water molecule and symmetry-related cation. The Co-N bond trans to Co-S is lengthened relative to the cis Co-N(en) lengths by 0.021(3) Ǻ. The complex is the only tractable product from the reaction of excess acetic anhydride in dimethyl sulfoxide with [Co(en)₂(SCH₂CO₂)]⁺ ions. The mechanism of formation is discussed.