AbstractLaser flash photolysis of diphenyliodonium salts produces phenyliodinium radical cation (PhI+·), which was also generated independently by flash‐induced electron transfer from iodobenzene to a phenanthrolinium salt. Apparent second‐order rate constants were determined for reaction of the transient (PhI+·) with nucleophiles, including iodobenzene and cyclohexene oxide. Quantum yields of formation of acid from stationary photolysis of diphenyliodonium hexafluoroarsenate were found to be significantly higher than yields of iodobenzene. These results may be explained by facile reaction of PhI+·with PhI to yield a new iodonium salt together with a proton. High reactivity of PhI+·with cyclohexene oxide suggests that the transient may directly initiate cationic polymerization of e