Abstract [Si,C,N,O] + and [Si,C,N,S] + cations formed upon electron ionization of trimethylsilyl isocyanate and isothiocyanate, respectively, are probed by tandem mass spectrometry. In the case of [Si,C,N,O] +, the experimental data suggest that the isocyanato group remains intact upon ionization such that silicon isocyanate SiNCO + is formed. Neutralization-reionization mass spectrometry indicates that also neutral SiNCO is a viable molecule in the gas phase. Instead, partial isomerization to SiSCN + is indicated for the sulfur analog. Because of this structural dichotomy, the nature of the [Si,C,N,S] + species giving rise to the recovery signal in the neutralization-reionization experiments remains uncertain.