Sulfur trichloride monoxide anion (SOCl3-) is stable in the gas phase. The G2 estimate of the energy for its dissociation to thionyl chloride and chloride anion is 102.8 kJ mol(-1). The potential energy surface for the reaction SOCl2 + Cl- --> SOCl3- has just a single well corresponding to the product. Collision-induced dissociation observed using a flowing afterglow-tandem mass spectrometer indicates a D-0(SOCl2-Cl-) bond energy of 85.4 +/- 7.6 kJ mol(-1), which is very similar to the previously determined bond energy D-0(SCl2-Cl-) = 85 kJ mol(-1).