Multication and multianion hybrid perovskites are among the most attractive materials currently under investigation for tandem photovoltaic applications because of the possibility they offer to finely tune the band gap, thus allowing the coupling with other semiconductors. If mixing different ions in the perovskite compositions can, on one hand, give the possibility to create different useful materials with different properties, on the other hand it could be detrimental to the hybrid perovskite stability. The presence of different ions leads to the formation of different polytypes that could open new pathways for the degradation of the prepared films. In this work, we have investigated the role of different polytypes on the degradation and stability of mixed hybrid perovskites by in situ X-ray diffraction analyses. We found that the use of even a small amount of methylammonium cations opens a new route for the degradation of the …