The decomposition of NH4ClO4 (AP) in both gaseous and condensed phases has been studied at the CCSD(T)/6-311 + G(3df,2p)//B3LYP/6-311 + G(3df,2p) level. Formation of NH3 + HClO3 is the main channel in the gas phase with 14.2 kcal/mol enthalpy change. In solution, the enthalpy change for this channel is 29.5 kcal/mol, which is close to the sublimation activation energy, ∼30 kcal/mol in crystalline AP. Formation of H3N-HO +ClO3 has higher enthalpy changes, 56.3 kcal/mol and 77.1 kcal/mol in the gas phase and in solution, respectively. Strong solvent effects may exist in the AP decomposition reaction.