Ammonium dinitramide (ADN) may serve as a 'green' oxidizer for rocket propellants since it provides chlorine free and minimum signature exhaust gases. However, stability issues and an erratic decomposition behaviour limit its non-questionable general usability. Nitric acid (HNO3) is a decomposition product of ADN and is known to accelerate the decomposition autocatalytically. In this study, we investigate the effect of HNO(3)on the initial decomposition steps of ADNviaquantum chemical calculations based on density functional theory (DFT)in vacuoas well as in an aqueous solution. We establish and compare the reaction profiles of ADN, its anion DN(-)and its acid HDN as well as their HNO3-clusters [NO3H_ADN], [NO3H_DN](-)and [NO3H_HDN] towards the products NO2, HNO(3)and N2O. All species exhibit various isomers with distinct decomposition pathways. We complement our study by investigating the protonated, cationic species [H_ADN](+), which may form in a low pH regime. The HNO(3)clusters reveal modulated energy barriers and in part altered decomposition pathways in comparison to the free DN-, HDN, and ADN species. The altered pathways include the formation of reactive intermediates (such as H2NO3+) as well as the formation of two HNO(3)molecules starting from one HNO(3)molecule, hinting at accelerated decomposition of ADN due to interaction with HNO3.