The gas-phase hydrolysis of N2O5 in the clusters of one to four water molecules is investigated by density functional theory calculations. The excess water molecules beyond stoichiometry of the reaction function as a polar solvent medium that stabilizes an ionic pair in the transition state. The reaction energy barrier is progressively diminished by the successive addition of water molecules and is completely eliminated in the cluster of four water molecules, permitting the hydrolysis reaction to proceed directly to products. Atmospheric implications are discussed to understand the reaction mechanisms for the hydrolysis of N2O5 in the gas-phase homogeneous conditions as well as in the heterogeneous conditions.