A theoretical study simulating the hypohalogenation reaction of cysteine (CSy) and N-acetylcysteine (NAC) has been performed with the objective of obtaining the energetic, electronic and kinetic properties for the reactions at room temperature and body temperature and considering neutral and basic conditions in aqueous medium. The study has been performed using the M06-HF, M06-2X, MPWB1K, BHANDHLYP density functional methods and with the MP2 method along with either the 6-311+G(d, p) basis set or the extended 6-311++G(3df,3pd) basis set. The results of the study indicate that all reactions proceed through the formation of the reactant complex structure, which is stabilised by the formation of intermolecular hydrogen bond between the reactant species. The reactions are also thermodynamically preferred to take place at room temperature. The reaction involving hypochlorous acid is faster and more thermodynamically preferred to the reaction involving hypochlorite anion, both at room temperature and at body temperature. The relative magnitude for the rate constant involving CSy and NAC is dependent on the pH of the solution. A comparison of the calculation methods utilised for the study suggests that the best methods for estimation of the rate constant are those containing both the HF-like exchange and the kinetic density components.