A sterically protected biradical species, 1-tert-butyl-3-methyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl was allowed to react with tris(4-bromophenyl)ammoniumyl hexachloroantimonate as an oxidant and the product was analyzed by EPR to show that the biradical was oxidized to give the corresponding radical cation species. The structure of the cation radical was also supported by ab initio calculations on a model compound.