In the field of photovoltaics, the mixed halide perovskite solar cells (PSCs) have been widely studied due to their excellent power conversion efficiency.[1] However, one of the obstacles of these PSCs is poor stability under ambient conditions such as light, moisture, air, etc. To improve the stability of PSCs, several inorganic cations have been incorporated into the mixed halide perovskite materials, of which Rb and Cs are two of the most effective cations.[2, 3] Nonetheless, the underlying mechanism of how those cations affect the morphology, structure, and composition, and factors affecting the stability of perovskite materials are still unclear and need to be adequately investigated. To achieve this, we conducted electron microscopy studies for Rb and Cs mixed halide perovskite materials. By using focused ion beam (FIB) combined with time of flight secondary ion mass spectrometry (ToF-SIMS), we have demonstrated that Cs+ cations are both segregated at the grain boundary and incorporated into the perovskite structure, while Rb+ cations only form a discrete Rb-rich phase at the grain boundary. In addition, the visibility of forbidden reflections in the electron diffraction patterns of the perovskite structure was also uncovered by electron diffraction studies. These forbidden reflections are proposed to be a direct result of the perovskite structure and can be attributed to three possible reasons. These include superlattice reflections resulting from a doubling of the unit cells, the chemical ordering within the A-site (Rb/Cs/MA/FA cations) and/or the halide site (I/Br anions), and twin formation. Furthermore, the coexistence of cubic (untilted PbI6 octahedra) and tetragonal (tilted PbI6 octahedra) phases in the perovskite structure at room temperature is also revealed. The insights presented in this study are essential to promote the research and development of the multi-cation mixed halide perovskite materials.