Y-Irradiation or anodic oxidation results in one-electron oxidation of trivalent-phosphorus compounds to give the corresponding trivalent-phosphorus cation radicals. Such cation radicals are generated also in chemical processes as reactive intermediates. Thus, trivalent-phosphorus compounds are converted to the intermediary cation radicals upon the reaction with good one-electron acceptors such as 10-methylacridinium ion in the photo-excited state, diazonium ion, and methylviologen. Trivalent-phosphorus cation radicals generated in these reactions easily undergo an ionic reaction with nucleophiles such as water and alcohol to afford phosphoranyl radicals, which subsequently decompose to the final products. Recently, kinetic examination on the reaction of phosphines with methylviologen has been presented, in which reactivity of trivalent-phosphorus cation radicals toward nucleophiles has been evaluated quantitatively. On the other hand, trivalent-phosphorus cation radicals exert radical-like reactivity toward a radical species. In order to interpret such duality in the reactivity of these cation radicals, dediazoniation of arenediazonium ions with trivalent-phosphorus compounds has been investigated on a mechanistic basis. It has been shown that relative ease of the ionic and the radical reactions of trivalent-phosphorus cation radicals is determined by spin density on the central phosphorus atom.