The tautomerism of all possible forms of 5-aminotetrazole (AT1-AT7) in the gas phase and continuum solvent was studied theoretically. The calculations were separately performed at the MP2 and CAM-B3LYP levels of theory, using the 6-311++ G(d, p) basis set. It was found that 5-aminotetrazole in the 2H form (AT1) is the most stable isomer in both the gas phase and in solution. In addition, the aggregation of various isomers of 5-aminotetrazole with hydrazine was investigated in the gas phase and in solution. Finally, the standard enthalpy of formations of the different structures of hydrazinium 5-aminotetrazolate was determined in the gas phase. Using the calculated standard enthalpy of formation of different structures of hydrazinium 5-aminotetrazolate in the gas phase and considering their Boltzmann population ratios, a single value has been reported for the standard enthalpy of formation of hydrazinium 5-aminotetrazolate considering all of the tautomers. The calculated values are in excellent agreement with the experimentally reported heat of formation for the hydrazinium 5-aminotetrazolate.