Thermolyses of N,N,N′,N′-tetramethylethylenediamine (1a), N,N,N′,N′-tetraethylethylenediamine (1b) and sym-N,N′-dimethylethylenediamine (1c) at 760–825 °C have been studied by ultraviolet photoelectron spectroscopy. Although the corresponding N-alkylated aminomethylene radicals were not observed, this study establishes that thermolysis of 1a is an efficient route to N-methylenimine (3a); methane, ethane, and ethene are the other major products. Diamine 1b yields, besides ethane, ethene, and propane, heretofore unreported N-ethylmethylenimine (3b). Diamine 1c yields imine 3a and methylenimine (3c), as well as hydrogen, methane, ethane, and ethene. Molecular orbital eigenvalues of the imines are calculated using HAM/3, MNDO, HF/STO-3G, HF/3-21G, and HF/6-31G* methods.