In contrast to , which is an efficient commercial phosphor, isostructural shows poor luminescence efficiency under host excitation at room temperature. In this work, the reason for this difference has been studied by optical measurements of solid solution . As is already known, the charge transfer band of Eu3+ is located below the absorption edge in . Meanwhile, in In‐rich solid solutions, the change transfer band is found inside the fundamental absorption region. This means that Eu3+ forms a deep electron‐trapping level in , but not in or In‐rich solid solutions, which gives a qualitative explanation for the luminescence quenching with increasing In concentration. More basically, it is considered that the difference in third ionization potentials of In, Eu, and Y results in different impurity levels of Eu3+ relative to the conduction band minimum. When the temperature is lowered, Eu3+ luminesces with a higher efficiency, particularly at high In concentrations. The temperature dependence of the luminescence intensity indicates that the energy is supplied to Eu3+ via a shallow unknown level. In this way, it was found that two different excitation paths are possible depending on the temperature.