Thermochemistry, kinetics, and mechanism of thermal decomposition of 1,5-diaminotetrazole (DAT), a widely used "building block" of nitrogen-rich energetic compounds, were studied theoretically at a high and reliable level of theory (viz., using the explicitly correlated CCSD(T)-F12/aug-cc-pVTZ procedure). Quantum chemical calculations provided detailed insight into the thermolysis mechanism of DAT missing in the existing literature. Moreover, several contradictory assumptions on the mechanism and key intermediates of thermolysis were resolved. The unimolecular primary decomposition reactions of the seven isomers of DAT were studied in the gas phase and in the melt using a simplified model of the latter. The two-step reaction of N2 elimination from the diamino tautomer was found to be the primary decomposition process of DAT in the gas phase and melt. The effective Arrhenius parameters of this process were calculated to be Ea = 43.4 kcal mol-1 and log(A/s-1) = 15.2 in a good agreement with the experimental values. Contrary to the existing literature data, all other decomposition channels of DAT isomers turned out to be kinetically unimportant. Apart from this, a new primary decomposition channel yielding N2, cyanamide, and 1,1-diazene was found for some H-bonded dimers of DAT. We also determined a reliable and mutually consistent set of thermochemical values for DAT (ΔfHsolid 0 = 74.5 ± 1.5 kcal·mol-1) by combining theoretically calculated (W1 multilevel procedure along with an isodesmic reaction) gas phase enthalpy of formation (ΔfHgas 0 = 100.7 ± 1.0 kcal·mol-1) and experimentally measured sublimation enthalpy (ΔsubH0 = 26.2 ± 0.5 kcal·mol-1).