In the present work we describe the process of formation of methylammonium lead iodide perovskite (MAPbI3) by reaction between [001] oriented PbI2 thin films and MAI. The reaction leads to a rapid change in the optical properties with the formation of a brown phase having a large band in the visible range and a contribution in the near-UV range (375 nm). X-ray diffraction analyses indicate that the MAI + PbI2 as-obtained material mainly consists of a highly oriented matrix of MAI that intermixes with the Pb–I precursor. A chemical exfoliation of the initial PbI2 structure with the massive intercalation of the methylammonium iodide ions suitably describes the optical and structural findings. Short thermal treating (100 °C) of this as-obtained sample results in the formation of a second oriented intermediate phase, attributed to the presence of a low-dimensional perovskite structure. Such low-dimensional metastable …