Reactions of oxidizing radicals like hydroxyl (.OH) radical, specific electron trans-fer agents like N^.₃,and I₋₂ radicals were studied with selenourea (SeU) and com- pared with thiourea (ThU) using pulse radiolysis technique in microsecond timescales. Both the compounds efficiently react with .OH radicals, however, SeU un-dergoes easier oxidation by secondary oxidants as compared to ThU. The results were supported by cyclic voltammetry studies. The radical cations of both SeU and ThU formed on oxidation undergo dimerization with the parent molecule toform two-centered three-electron-hemi bonded radical cations absorbing at 410 and 400 nm respectively with the stabilization energies of 21.1 and 20.5 kcal/mol for SeU and ThU, respectively. Preliminary studies indicated that at low concentration of SeU, the dimerization is prevented and the oxidation reaction produced metallic Se nanoparticles.