A combined experimental and theoretical study is employed to elucidate one- electron oxidation mechanism of methionine methyl ester (MME) in aqueous solution. In neutral solution, (OH)-O-center dot radical reaction leads to the formation of a monomer transient species absorbing at 380 nm and a dimer transient species absorbing at 480 nm is formed in acidic solution. Quantum chemical calculations suggest that the transient species formed on reaction of (OH)-O-center dot radicals in neutral solution has 5-member ring structure. The transient absorption band formed on reaction of Cl-2(center dot-) or Br-2(center dot)- with MME is shown to be due to hemi bonded adduct between Cl or Br atoms and S in MME. (c) 2005 Elsevier B.V. All rights reserved.