[Si,C,N,O](+) and [Si,C,N,S](+) cations formed upon electron ionization of trimethylsilyl isocyanate and iosthiocyanate, 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.