Behavior of radicals produced in the photochemicalreaction of 9,10-phenanthrenequinone was investigated by use of CIDNP and ESR technique. By comparison of the sign of CIDNP signals in formation of and in decomposition (photochemical or thermal) of adducts, the rearrangement of adducts, e.g., from the 1,2-adduct to 1,4-adduct and vice versa, was proposed. Although the adducts are stable enough to