AbstractThe measurement of quantum yield (QY) is an important process for a phosphor. This is often accomplished by the comparison of the low‐temperature lifetime with the room temperature (RT) lifetime. It is shown that this procedure provides a considerable underestimation of QY in certain cases. Additionally, the use of the europium ion as an indicator of changes in site symmetry with temperature has severe limitations. This work reveals that a decrease in measured lifetime with increasing temperature of a system cannot always be associated with a descent in site symmetry and may be due to increasing radiative rate.