Recently, hybrid organic-inorganic metal halide perovskites have gained prominence as potent light harvesters in thin film solid-state photovoltaics. In particular the solar-to-electric power conversion efficiency (PCE) of devices using CH3NH3PbI3 as sensitizer has increased from 3 to 20.1% within only a few years. This key material can be prepared by solution processing from PbI2 and CH3NH3I in one step or by sequential deposition. In the latter case an electron capturing support such as TiO2 is first covered with PbI2, which upon exposure to a CH3NH3I solution is converted to the perovskite. Here we apply for the first time quartz crystal microbalance (QCMD) measurements in conjunction with X-ray diffraction and scanning electron microscopy to analyse the dynamics of the conversion of PbI2 to CH3NH3PbI3. Employing 200 nm thick PbI2 films as substrates we discover that the CH3NH3I insertion in the PbI2 is reversible, with the extraction into the solvent isopropanol occurring on the same time scale of seconds as the intercalation process. This offers an explanation for the strikingly rapid and facile exchange of halide ions in CH3NH3PbX3 by solution processing at room temperature.