Cysteine oxidation by HO* was studied at high level of the ab initio theory in both gas phase and aqueous solution. Potential energy surface scans in gas phase performed for the model system methanethiol+HO* indicate the reactants can form two intermediate states: a sulfur-oxygen adduct and a hydrogen bound reactant complex. However these states appear to play a minor role in the reaction mechanism as far as they are fast dissociating states. Thus the main reaction channel predicted at the QCISD(T)/6-311+G(2df,2pd) level of theory is the direct hydrogen atom abstraction. The reaction mechanism is not perturbed by solvation that was found to induce only small variations in the Gibbs free energy of different reactant configurations. The larger size reactant system cysteine+ HO* was treated by the IMOMO hybrid method mixing the QCISD(T)/6-311+G(2df,2pd) and UMP2/6-311+G(d,p) levels of theory. The calculated potential energy, enthalpy and Gibbs free energy barriers are slightly different from those of methanethiol. The method gave a rate constant for cysteine oxidation in aqueous solution k=2.4x109 mol-1 dm3 s-1 that is in good agreement with the experimental rate constant. Further analysis showed the reaction is not very sensitive to hydrogen bonding and electrical polarity of the molecular environment.