Coordination polymers and metal-organic frameworks can be modified by high pressure, according to its effects on the radii of central and ligand atoms. The pressure reduces the ligands' radii, and the coordination number is usually increased. Such transformations of the coordination quite generally conform to the inverse rule of pressure and temperature effects, although the temperature-induced transformations are much less frequently observed. The two-dimensional coordination polymer Cd(APP)(2)NO3 center dot NO3 [APP = 1,4-bis(3-aminopropyl)-piperazine] undergoes a pressure-induced isostructural phase transition triggered by a topochemical reaction, yielding Cd(APP)(2)(NO3)(2) . The transition retains the symmetry of both phases, and their structures have been determined by X-ray diffraction for the single crystals compressed in a diamond-anvil cell. The reaction increases the Cd coordination, from 6-fold in phase I to 7-fold in phase II, where the new Cd-O bond involves an additional nitrate anion in the Cd coordination sphere.