A continuously flowing (CH3)(2)CHN beam is generated by pyrolysis of (CH3)(2)CHN3 at 113(+/-0.5)degreesC using normal inlet system with an 8 mm bore of the exit of the quartz tube under the presence of molecular sieve (30 Angstrom) 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)C=NH, which comes from the isomerization of (CH3)(2)CHN. The ionization energies of (CH3)(2)CHN and (CH3)(2)C=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)(2)CHN is a diradical with C-s symmetry and has a (3)A" ground state, and (CH3)(2)C=NH is a closed shell molecule with C-s symmetry. (C) 2003 American Institute of Physics.