One-electron oxidation of selenourea in aqueous solution has been studied using pulse radiolysis, cyclic voltarnmetry, and ab initio quantum chemical techniques. Specific one-electron oxidants (N-3(.) Br-2(.), CI2.-, I-2(.-)) react with selenourea to form a broad transient optical absorption band with the maximum absorption at the wavelength (lambda(max)) of 410 nm. The hydroxyl radical reacts with selenourea at a bimolecular rate constant of k = 9.9 x 10(9) M-1 s(-1) and a similar transient band at lambda(max) = 410 nm is formed. The absorbance at 410 nm has been observed to be dependent on solute concentration. The transient band at 410 nm is assigned to the selenourea dimer radical cation. The transient shows reactivity toward oxygen (k = 8.6 x 10(7) M-1 s(-1)). The H-. atom is observed to react with selenourea with a bimolecular rate constant of 2.1 x 10(9) M-1 s(-1), giving a transient optical absorption band at 410 nm, which is also assigned to the same dimer radical cation formed following, the H-abstraction reaction. Redox reaction studies in aqueous solution by pulse radiolysis and cyclic voltarnmetry revealed that the one-electron oxidation potential of selenourea is less than that of its lighter analogues, thiourea and urea. The theoretical result that is based on the ab initio quantum chemical method confirms that the transient optical absorption band at 410 nm is due to the dimer radical cation of selenourea. The calculation, which is based on nonlocal correlated hybrid density functional theory, illustrates the formation of an intermolecular two-center, three-electron (2c-3e) bond between two Se atoms with a binding energy of 21.1 kcal/mol.