The He I and He II photoelectron spectra of two proazaphosphatranes were recorded and interpreted by correlated ab initio quantum chemical calculations. The first ionization energy of the compounds investigated is among the lowest reported hitherto for phosphorus compounds. Geometry optimization of the ionic states of the parent proazaphosphatrane (at the HF and MP2 levels) revealed that there are two minima on the potential energy surface at different P-N-ax distances representing two bond-stretch isomers. The structure with the long P-N-ax distance (3.3 Angstrom) is similar to that of the neutral molecule and thus has a favorable Frank-Condon factor for the ionization process. The other structure, with a short P-N-ax distance (2.07 Angstrom), is as much as 1 eV lower in energy than the other. Due to the unfavorable Frank-Condon factor, however, the corresponding adiabatic ionization energy cannot be seen in the spectrum. The short P-N-ax distance in the more stable bond-stretch isomer is characteristic for azaphosphatranes. Its stability can thus be related to the formation of the dative P <-- N bond. The unobserved adiabatic ionization energy should be about 1 eV lower than that of the observed one (ca. 5.6 eV), in full agreement with the strong basicity of proazaphosphatranes.