Oxidation reactions of DL-selenomethionine (DL-SeMet) with hydrogen peroxide and ozone were investigated by H-1 and C-13 NMR spectroscopy. Chemical shifts of the reaction products were measured at different pD values in D2O solutions. In the moderate (4-8) pD range, a pair of singlets of equal intensity appeared at 2.71 and 2.80 ppm, with the disappearance of the DL-SeMet methyl peak at 2.04 ppm. As the pD of the solution decreased, the chemical shifts of both peaks increased relative to 3-trimethylsilylpropionic acid-d(4) sodium salt. In strongly acidic solutions (pD < 3), the pair collapsed to one singlet at 3.18 ppm. As pD values increased the chemical shifts decreased in numerical value with only a singlet at 2.69 ppm observed in strongly basic solutions (pD > 10). These data were assigned to Met selenoxide in basic solution, stabilized by intermolecular hydrogen bonding, and to the hydration of Met selenoxide to Met dihydroxyselenide in acidic solutions. In moderate pD solutions (pD = 4-8), both forms of the selenoxide can exist. For the reaction of DL-SeMet with CNBr, two parallel reactions occur, i.e. oxidation and bond cleavage with cyanization : the bond cleavage giving the expected products, i.e. 2-amino-4-butyrolactone and methyl selenocyanate, with an additional singlet observed at 2.54 ppm.