Collisional activation of keV thiophene-2-one radical cations 1(+.) with O-2 or NO. as the target gas leads to a desulfuration reaction. This peculiar reaction is insignificant or absent with other targets such as helium, argon, methane or nitrogen. The radical cations produced in this desulfuration reaction are most probably vinylketene ions, as indicated by a triple mass spectrometric (MS/MS/MS) experiment performed on a 'hybrid' tandem mass spectrometer of sector-quadrupole-sector configuration. Tentatively, it is proposed that population of an excited state accounts for the non-ergodic behavior of 1(+.) upon collision with oxygen or nitric oxide. Ab initio molecular orbital calculations using molecular orbital theory (UMP2, UCCSD(T)) and density functional theory (B3LYP) with 6-31G(d,p) and 6-311++G(d,p) basis sets were used to evaluate the relative energy of the excited quartet state of 1(+.) radical cations. This quartet state is calculated to lie about 3.6 eV above the (2)A " ground state and 0.9 eV above the C4H4O+. + S dissociation products. It is proposed that the quartet ion serves as the precursor for the spontaneous desulfuration. Copyright (C) 2001 John Wiley & Sons, Ltd.