Photosensitive substance of AgBr 1− x I x (Fm3m, Z = 4) mixed crystal was prepared with grain size of 0.8–2.0 μm in diameter and it is composed of a zonal structure of which each layer has a different composition between x = 0.03 and 0.30. X-ray diffraction profile analysis of the mixed crystal has been made by using synchrotron radiation and it reveals lattice distortion and thermal diffuse scattering. The full width at half maximum of the diffraction profile continuously increases with AgI content of the solid solution up to x = 0.30, because a larger anion I induces the lattice distortion. A pure AgBr specimen shows a long tail of the diffraction profile, because it is characterized by a large ion conduction, resulting in the remarkable diffuse scattering. A linear relation between the lattice constant and AgI concentration is represented by a( X) = 0.57748 + 0.000368 X (nm).