Ab initio MO calculations at the QClSD(T)/6-311++G(3df,2p) + ZPE level show that the thionitrosyl chloride ion, ClNS+., is 36 kJ/mol more stable than the thiazyl chloride ion, NSCl+., whereas neutral NSCl is 77 kJ/mol more stable than ClNS [IE(a) (ClNS) = 9.2 +/- 0.3 eV, IE(a)(NSCl) = 10.5 +/- 0.3 eV]. Mild flash-vacuum pyrolysis of the thiazyl chloride trimer (NSCl)(3) followed by electron impact ionization resulted in the formation of [N,S,Cl](+.) ions (m/z 81). The fragments observed in the CA spectrum of these ions indicate the formation of both NSCl+. and ClNS+. ions. A very weak recovery signal is observed in a neutralization-reionization experiment. This signal is tentatively assigned to the neutral thionitrosyl chloride.