Gaseous trans-Bu'P=NBu', generated by mild gas-phase thermolysis of its more stable [2 + 11 cyclodimer, has been characterized by field-ionization mass spectrometry and U.V. photoelectron spectroscopy. The photoelectron spectrum has been assigned based on SCC-Xa model calculations for representative H E=EH and E, molecules (E, = N,, PN, or P,) and by comparison with the known ionization potentials of R N N R and E, species. The low-energy sequence for parent dial kyldi p hosphenes is deduced.