The P− T phase diagram associated with the ferromagnetic semiconductor CdCr 2Se 4 was determined between 600 and 964°C for the first time from the P Se 2(or Cd) − T diagram for the stability of the compound disclosed by annealing experiments under controlled Se 2 and Cd vapor pressures, and from the phase equilibria in the CdSeCrSeSe system examined by usual annealing experiments. The following characteristic features are also clarified. (1) The maximum temperature for the stability of CdCr 2Se 4 is about 900°C. (2) CdCr 2Se 4 dissolves into Se melt and is in equilibrium with the Se-rich melt which has a CdSe content in excess of CdSe·Cr 2Se 3. The solubility decreases rapidly as temperature falls from 900 to 860°C. (3) Cr 2Se 3 is the only compound in the CrSe system in equilibrium with CdCr 2Se 4 above about 860°C. Both Cr 3Se 4 and Cr 2Se 3 exist below this temperature. (4) The P Se 2 range for the stability of CdCr 2Se 4 is estimated as a measure of the concentration ranges of native defects due to nonstoichiometry. It increases as temperature decreases and is about 5 × 10 9 at 600°C.