The coupling of flash vacuum thermolysis reactions (retro [2+2] cleavages) with photoelectron spectroscopy allowed the first direct characterization of several highly reactive N-alkylated silanimines. For the trimethylsilanimine 1b, the IPs corresponding to the ejection of an electron from the nitrogen lone pair (n(N)) and the pi(Si)=N orbital are observed at 7.9 and 8.3 eV, respectively. In the case of the N-isopropylated and N-tert-butylated analogues 1c,d, these characteristic ionizations are energetically closed (7.8 and 7.4 eV). These unexpected attributions have been corroborated by the ab initio calculations of the ionization energies using 6-31G(d) and LANL1DZ basis sets and MP2 level. Our results have been completed by the determination of the structural and electronic characteristics of these silanimines.