Significance Organic–inorganic hybrid perovskites are exciting new solar cell materials. In 2D perovskites, the 3D structure is spatially interrupted by a layer of intercalated ions, which may alter both stability and functionality. A major question concerns the limit to which the 2D architecture can be maintained as the thickness of the layers increases. A combination of synthetic chemistry, crystallography, and spectroscopy was used to obtain and identify the thickest 2D halide perovskite characterized to date, having seven layers. Experimentally measured enthalpies of formation as a function of layer thickness show systematic behavior and that higher homologues are energetically unstable, placing a limit on the number of layers possible in a single-phase 2D perovskite. Their application in photovoltaics is discussed.