X-ray diffraction and DSC investigations indicate that poly(dipropylphosphazene) is characterized by one ordered crystalline modification and a thermotropic mesophase, which is formed at temperatures higher than 248 degrees C and appears to maintain a 2D order. The structure of the crystalline phase has been determined. the unit cell is monoclinic, space group P2(1)/c, and the cell parameters are: a = 9.10(5) Angstrom, b = 4.88(4) Angstrom, c = 19.05(10) Angstrom, beta = 92.43(1)degrees. The refinement indicates a twofold helical conformation with a nearly planar (CT)(2) backbone. The cyclic trimer was studied by DSC and single crystal X-ray diffraction: the unit cell is monoclinic, space group P2/c, cell parameters are 10.0131(5) Angstrom, b = 7.8342(4) Angstrom, c = 15.1782(11) Angstrom, beta = 95.844(7)degrees The conformations of the side chains in the polymer and the trimer are closely related, as already found in polydiethylphosphazene. In the polymer, however, the side chains probably show some disorder yet to be characterized.