Pulse radiolysis studies on hydroxyl ((OH)-O-center dot) radical reactions of selenomethionine (SeM), a selenium analogue of methionine, were carried out, and the resultant transient radical cations and their subsequent reactions have been reported. At pH < 3, the > Se-center dot-OH radical adducts produced on reaction of SeM with (OH)-O-center dot radical were converted to selenium centered radical cations (Se center dot+M), which react with another molecule of SeM to form dimer radical cation M(Se therefore Se)M+.At pH 7, the > Se-center dot-OH radical adducts were converted to a monomer radical of the type (Se therefore N)M+ that acquires intramolecular stability through interaction with the lone pair of the N atom and this radical is denoted as SeM center dot+.SeM center dot+ decayed by first order kinetics, and the reduction potential of the couple SeM center dot+/SeM was determined to be 1.21 +/- 0.05 V vs NHE at pH 7. SeM center dot+ oxidized ABTS(2-) and TMPD with rate constants of (2.5 +/- 0.1) x 10(8) and (6.1 +/- 0.2) x 10(8) M-1 s(-1), respectively, and reacted with hydroxide ion with a rate constant of (3.8 +/- 0.9) x 10(5) M-1 s(-1). SeM center dot+ reacts with molecular oxygen, and the rate constant for this reaction was determined to be (4.3 +/- 0.2) x 10(8) M-1 s(-1); similar reaction with methionine could not be observed experimentally. Like methionine radical cations, SeM center dot+ undergoes decarboxylation, although with lesser yield, to produce reducing 3-methyl-selenopropyl amino radicals (referred as alpha-amino radicals). The formation of these radicals was confirmed both by the estimation of the liberated CO2 and by one-electron reduction of MV2+, thionine, and PNAP. These results have been supported by quantum chemical calculations. Implications of these results in the biological role of SeM have also been briefly discussed.