The structural features of poly[bis(4-isopropylphenoxy)phosphazene] (PB(4-ip)PP) have been studied by electron microscopy, X-ray diffraction and differential scanning calorimetry techniques. Its orthorhombic lattice constants are determined as follows: a = 3,14 nm, b = 1,14 nm, c = 0,992 nm. The space group of this polymer is suggested to be P 2(1)2(1)2(1) - D-2(4), and the molecular conformation of the chains possibly to be (trans(3)cis)(2). This polymer exhibits a crystal/crystal transition at 86 degrees C below its T(1) transition (120 degrees C). The thermal behavior is similar to the characteristics of poly[bis(p-methoxyphenoxy)phosphazene].