Chemical preparation, thermal behavior and IR studies are given for the cyclotriphosphate CdK4 (P3O9)2.2H2O and its anhydrous form CdK4 (P3O9)2. CdK4 (P3O9)2.2H2O, isotypic to MnK4 (P3O9)2.2H2O, is triclinic P-1 with the following unit-cell dimensions: a = 9.235(4)Å, b = 7.599(3)Å, c = 7.148(1)Å, = 96.38(2)°, = 103.90(5)°, = 102.06(3)° and Z = 1. The total dehydration of CdK4 (P3O9)2.2H2O, between 350°C and 420°C, leads to its anhydrous form CdK4 (P3O9)2. CdK4 (P3O9)2 is a new cyclotriphosphate crystallizing in the rhombohedral system, space group P-31c, Z = 2 with the following unit-cell dimensions: a = b = 7.337(3)Å and c = 19.920(1)Å. CdK4(P3O9)2 is stable until its melting point at 431°C. The thermal behavior of CdK4(P3O9)2.2H2O has been investigated and interpreted by comparison with IR absorption spectrometry and X-ray diffraction experiments. Two different methods Ozawa and KAS have been selected in studying the kinetics of thermal behavior of the title compound.