Abstract The existence of three main crystalline phases (called III, II and I) in (C 12H 25NH 3) 2CdCl 4 has been revealed by differential scanning calorimetry. X-ray diffraction and spectroscopic studies. The crystal- lographic evolution with increasing temperature appears to be monoclinic (III) → orthorhombic (II) → tetragonal (I). The low temperature phase III is the only ordered structure. The phase transition (III-II), which is of first order type, corresponds to an order-disorder mechanism involving the organic part of the structure (alkylammonium chains) whereas the phase transition (II-I), which is of second-order type, is related to the arrangement of the mineral matrix (octahedra of perovskite layers). An intermediate disordered form II', stable in a very narrow temperature range and structurally similar to the form II, has also been observed, so that the transformation (III-II) proceeds, in fact, in two steps (III-II'-II). The variation enthalpies observed at the transitions (III-II'-II) and analyzed through an order-disorder mechanism demonstrate the high disorder of the alkylammonium chains in form II, in agreement with spectroscopic results. No thermal anomaly or spectroscopic modification is observed for the high temperature transition (II-I).