A theoretical investigation at the MP2/6-311++G(2d,2p)//MP2/6-31+G(d,p) level was employed in order to study the one-electron oxidation mechanism of methionine in aqueous solution. Three reaction paths corresponding to the HO center dot, O-2 attack and hydrolysis were considered. Results show that all the processes are exothermic and that the rate determining step can be associated with the hydrolysis reaction. DFT computations with different exchange-correlation potentials were performed in order to verify their reliability in the description of the cyclic adduct produced in the HO center dot attack step.