The optical and structural properties of hybrid perovskites can be tuned by the post-synthetic introduction of new cations. To advance the development of this approach, knowledge of the reaction mechanism is essential, but has not yet been elucidated. Here, the effect of n-octylamine on three-dimensional (3D) methylammonium lead bromide (MAPbBr3) was investigated by in situ X-ray photoelectron spectroscopy. Spectroscopic analysis indicated equimolar substitutions between octylammonium (OcA+) and methylammonium (MA+) cations that cause the formation of two-dimensional (2D) octylammonium lead bromide ((OcA)2PbBr4). The introduction of methylamine reversed these changes, and the cation exchange between MA+ and OcA+ caused the reverse conversion to MAPbBr3.