The density functional theory and ab initio methods are used to study two minimum energy pathways for the hydrolysis of N 2 O 5 to form nitric acid, one involving a single water molecule and the other involving two water molecules. Both of the reactions are initiated by the nucleophilic attack on N 2 O 5 by water while the additional water in the two-water reaction acts as a polar solvent to stabilize the ion pair resulted from the nucleophilic attack. The activation energy is 20 kcal/mol for the one-water reaction and it is reduced by half for the two-water reaction. The study suggests that the heterogeneous hydrolysis of N 2 O 5 is likely more favorable than the homogeneous process.