We will use X-ray photon correlation spectroscopy to reveal the reversible dynamics of the light-soaking domains in hybrid organic-inorganic perovskites. These domains appear due to light-induced segregation of iodide and bromide ions in MAPb(I/Br)3 films and are characterized by different band gaps and slightly different crystal structures. Using the scattering of coherent X-rays in Bragg geometry, we will characterize the size of the iodine-rich and bromide-rich domains and obtain a characteristic time scale of their formation. This information will help to improve the stability of the perovskite-based solar cells. These kind of experiment can be applicable rather generally to study the ion migration in various soft- and hard-condensed matter systems.