Abstract ESR study was executed on the mixed crystal of rubidium silver iodide and ammonium silver iodide. In (Rb0.9(NH4)0.1)2AgI3 γ-rayed at 77 K, I2 − was on the horizontal edge of the iodine tetrahedron that chained by each other in the direction parallel with the b-axis of the crystal. I0 was on one of the iodine ion on the horizontal edge of the iodine tetrahedron and slightly distributed to one of near ammonium ions. Their hyperfine parameters were approximately the same as those in the ammonium silver iodide or akali silver iodides. I2 − : |A∥| = 42.0 mT, |A⊥l = 19.5 mT, g∥ = 1.91, g⊥ = 2.57. I0 |A∥| = 51.0 mT, |A∥| = 21.0 mT, g⊥ = 1.99, g⊥ = 2.26. The I2 − and I0 were thermally more stable than those in the each component crystal.