In this abstr. 1-15 = I-XV. The photoelectron spectra of substituted tetrazoles 1-3, 1,4-dihydro-5H-tetrazol-5-ones 4-9, and 1,4-dihydro-5H-tetrazole-5-thiones 10-15 have been recorded. Based on PM3 and some ab initio calcns., the ionization potentials have been assigned to MOs. Gas-phase thermolyses of 1-15 have been studied by real-time gas anal. controlled by photoelectron spectroscopy. Compds. 1 and 2 lose formaldehyde and thioformaldehyde, resp., leaving unsubstituted tetrazole, which decomps. mainly through extrusion of a nitrogen mol. and formation of cyanamide. Thiirane is split off from 3, and the remaining mol. decomps. into smaller products. Compds. 4-9 are cleaved by [3+2] cycloreversion to isocyanates and azides. Some of the unsym. substituted compds. exhibit a marked selectivity in this reaction. For thiones 10-15 [3+2] cycloreversion is the main pathway of decompn., affording isothiocyanates and azides. In addn., the sulfur atom can split off and dimerize or abstr. hydrogen atoms to form hydrogen sulfide. Some products like thiirene, formaldehyde, thioformaldehyde and acetaldehyde are generated solely from substituents. Photoelectron spectroscopy proved to be an excellent method for such thermolysis studies.