We investigate the degradation path of MAPbI 3 (MA= methylammonium) films over flat TiO 2 substrates at room temperature by means of X-ray diffraction, spectroscopic ellipsometry, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The degradation dynamics is found to be similar in air and under vacuum conditions, which leads to the conclusion that the occurrence of intrinsic thermodynamic mechanisms is not necessarily linked to humidity. The process has an early stage, which drives the starting tetragonal lattice in the direction of a cubic atomic arrangement. This early stage is followed by a phase change towards PbI 2. We describe how this degradation product is structurally coupled with the original MAPbI 3 lattice through the orientation of its constituent PbI 6 octahedra. Our results suggest a slight octahedral rearrangement after …