Thionyl diisocyanate which is unstable at ambient temperature is generated from a heterogeneous reaction of gaseous thionyl dichloride with silver cyanate and studied for the first time in the gas phase at 298 K. N-isocyanatoformyl sulfinylimide is also observed with photoelectron spectroscopy in the gas phase for the first time. The conversion of thionyl diisocyanate to N-isocyanatoformyl sulfinylimide via 1,3 shift at different temperatures is studied by using photoelectron spectroscopy and mass spectroscopy with the help of theoretical computations. On the basis of combined observations and quantum chemical calculations, possible 1,3-isocyanato shifts and dissociation processes for thionyl isocyanate and its cation have been discussed in detail.