Pulse radiolysis coupled with absorption detection has been employed to study one-electron oxidation of selenomethionine (SeM), selenocystine (SeCys), methyl selenocysteine (MeSeCys), and selenourea (SeU) in aqueous solutions. Hydroxyl radicals ((OH)-O-center dot) in the pH range from 1 to 7 and specific one-electron oxidants Cl-2(center dot-) (pH 1) and Br-2(center dot-) (pH 7) have been used to carry out the oxidation reactions. The bimolecular rate constants for these reactions were reported to be in the range of 2 x 10(9) to 10 x 10(9) M-1 s(-1). Reactions of oxidizing radicals with all these compounds produced selenium-centered radical cations. The structure and stability of the radical cation were found to depend mainly on the substituent and pH. SeM, at pH 7, produced a monomer radical cation (lambda(max) similar to 380 nm), while at pH 1, a dimer radical cation was formed by the interaction between oxidized and parent SeM (lambda(max) similar to 480 nm). Similarly, SeCys, at pH 7, on one-electron oxidation, produced a monomer radical cation (lambda(max) similar to 460 nm), while at pH 1, the reaction produced a transient species with - 560 nm), which is also a monomer radical cation. MeSeCys on one-electron oxidation in the pH range from 1 to 7 produced monomer radical cations (lambda(max) similar to 350 nm), while at pH < 0, the reaction produced dimer radical cations (lambda similar to 460 nm). SeU at all the pH ranges produced dimer radical cations (lambda(max) similar to 410 nm). The association constants of the dimer radical cations of SeM, MeSeCys, and SeU were determined by following absorption changes at as a function of concentration. From these studies it is concluded that formation of monomer and dimer radical cations mainly depends on the Substitution, pH, and the heteroatoms like N and O. The availability of a lone pair on an N or O atom at the beta or gamma position results in monomer radical cations having intramolecular stabilization. When such a lone pair is not available, the monomer radical cation is converted into a dimer radical cation which acquires intermolecular stabilization by the other selenium atom. The pH dependency confirms the role of protonation on stabilization. The oxidation chemistry of these selenium compounds is compared with that of their sulfur analogues.