We study by density functional theory the hydrolysis of a chlorambucil analogue. Three SN1 and one SN2 mechanisms have been compared. Results show that the most likely mechanism involves the formation of an aziridinium ion via a first-order reaction subject to an energy barrier of 24.8 kcal/mol. Additionally, a kinetic study, using the thermodynamic formulation of the Transition State Theory, has been carried out. Theoretical results coincide with experimental values obtained under similar conditions of pH, temperature and chloride concentration.