DOI: 10.1063/1.1577316 ; A continuously flowing (CH₃)₂CHN beam is generated by pyrolysis of (CH₃)₂CHN₃at 113(±0.5)°C using normal inlet system with an 8 mm bore of the exit of the quartz tube under the presence of molecular sieve (30 Å) and stabilizing NO gas, and its HeI photoelectron (PE) spectrum is also recorded in situ. A spectrum recorded further away from the pyrolysis catalyst or by using a 0.4 mm bore of the quartz tube is the PE spectrum of (CH₃)₂=NH, which comes from the isomerization of (CH₃)₂CHN. The ionization energies of (CH₃)₂CHN and (CH₃)₂C=NH are determined for the first time by the photoelectron spectroscopy experiment, and Gaussian 2 and improved density functional theory calculations. Experimental and theoretical results agree reasonably well, and show that (CH₃)₂CHN is a diradical with C[sub s] symmetry and has a³A" ground state, and (CH₃)₂C=NH is a closed shell molecule with C[sub s] symmetry. ; Author name used in this publication: D. K. W. Mok