The decomposition pressure of was measured by the torsion‐effusion method over the temperature range 723°–767°K. Mass spectrometric analysis showed the effusing vapor to be and that the decomposition/vaporization process was , in agreement with vapor molecular weight determinations. The observed vaporization process was not the thermodynamically most favored one, which would have produced gaseous . Observed pressures were found to be dependent on the area of the effusion orifice, and the results were extrapolated to zero orifice size to derive the following equation for the equilibrium pressures of