The room temperature structure of [C(NH 2 ) 3 ] 3 Sb 2 Cl 9 · 0.9 H 2 O (GNCA) was solved. It crystal lizes in the monoclinic C2/c space group with a = 15.275, b = 8.794, c = 17.904 (in Å), β = 96.40°, V = 2390 Å 3 , Z = 4. Refinement of the atomic parameters by a least squares methods gave R = 0.042, wR = 0.039 for 1958 reflections with F>4σ(F). The structure consists of polyanionic (Sb2Cl 9 3- ) n layers built of deformed corner connected SbX 6 3- octahedra. Two crystallographically inequivalent guanidinium cations are present, one situated between polyanionic layers, the other one together with a disordered water molecule inside cavities formed by polyanions. Temperature X-ray scattering experiments together with DSC studies were carried out above room temperature. Temperature dependence of the lattice parameters between 300 K and 380 K was determined and a phase transition of mixed order at 364 K was revealed.