The electronic properties and thermal decomposition of 5-methyltetrazole (5MTZ) are investigated using UV photoelectron spectroscopy (UVPES) and theoretical calculations. Simulated spectra of both 1H- and 2H-5MTZ, based on electron propagator methods, are produced in order to study the relative tautomer population. The thermal decomposition results are rationalized in terms of G2(MP2) results. 5MTZ yields a HOMO ionization energy of 10.82 ± 0.04 eV and the gas-phase 5MTZ assumes predominantly the 2H-form. Its gas-phase thermal decomposition starts at ca. 195 °C and leads to the formation of N 2,CH 3CN and HCN. N 2 is formed from two competing routes, involving 150.2 and 126.2 kJ/mol energy barriers, from 2H- and 1H-5MTZ, respectively. CH 3CN is formed also from two competing pathways, requiring activation energies of 218.3 (2H-5MTZ) and 198.6 kJ/mol (1H-5MTZ). Conclusions are also drawn in order to explain the formation of HCN from secondary reactions in the thermal decomposition process.