We will use GIWAXS and GISAXS to understand the structural changes in mixed halide perovskites APbX3 (A=methylammonium, formamidinium, cesium; X=Cl, Br, I). We will track the light-induced segregation of halides in I/Br and Br/Cl mixed perovskites. We will observe the formation of pure phase domains (i.e. I-rich, Br-rich or Cl-rich) and characterize their typical size and time scale over which they appear. We will study the influence of chemical composition and crystal grain size on halide migration. This information will help to improve the stability of the perovskite-based solar cells and give an insight into ion migration in perovskite structures. The samples will be prepared at ESRF using own portable spin coater which was already tested at ID10.