We performed the single-crystal X-ray diffraction study of a perovskite-type gold mixed-valence compound, Cs 2 Au I Au III Cl 6 , under high pressures up to 18GPa by using a diamond-anvil-cell with helium gas as an ideal hydrostatic pressure-transmitting medium. The lattice parameters and the variable atomic positional parameters were obtained with reasonable accuracy at various pressures. A structural phase transition at ca. 12.5GPa from I4/mmm to Pm3m was found. The lattice parameters a 0 and c 0 , denoted in the tetragonal cell setting, result in the relationship 2 1/2 a 0 =c 0 , and the superstructure reflections hkl (l is odd), caused by the shift of the Cl ions from the midpoint of the Au ions, disappeared at pressures above the phase transition. Both elongated [Au III Cl 6 ] and compressed [Au I Cl 6 ] octahedra in the low-pressure phase smoothly approach regular octahedra with increasing pressure. Above the structural phase transition at 12.5GPa, all the [AuCl 6 ] octahedra are crystallographically equivalent, which shows that the tetragonal-to-cubic phase transition accompanies the valence transition from the Au I /Au III mixed-valence state to the Au II single-valence state.