Crystals of N-propylurotropinium heptaiodide, containing unusual pyramidal [I₃^–·2I₂] moieties, have been subjected to controlled temperature and pressure changes. Thermogravimetric analysis revealed their complex thermochemistry at elevated temperature, which includes the formation of stable intermediate polyiodides. X-ray diffraction experiments on decomposition products elucidated the thermal breakdown pathways and the role of the crystalline environment on their thermostability. The electrical conductivity has been shown to drop significantly as the parent crystal is depleted of iodine. The thermostabilizing role of compression and its relation to the supramolecular architecture emerges from high-pressure synchrotron X-ray diffraction data. High pressure prevents the studied polyiodide from decomposition at high temperature and significantly raises its melting point. Additionally, compression fosters the stepwise catenation between diiodine and triiodide units, up to the formation of I₁₄^2- moieties at ca. 4 GPa. The observed effects were related to the pressure-induced changes in the crystal ionic strength and vibrational entropy, as well as the increase in I-I bond covalency.