The influence of impurities on the luminescence quantum efficiency of Ce0.2Gd0.6Tb0.2MgB5O10 has been studied. Low concentrations of Pr3+, Nd3+, Dy3+, Ho3+, and Er3+ do not compete efficiently with Gd3+ and Tb3+ for the excitation energy on the Ce3+ ions. However, the luminescence efficiency of Ce3+ is strongly decreased by Sm3+ and Eu3+ due to an electron transfer process. The luminescence efficiency of Ce3+ is also decreased by Cr2+, Mn2+, Fe2+, and Ni2+ due to energy transfer to these ions. In the case of Cr3+ the luminescence efficiency of Ce3+ is quenched due to competitive absorption. The luminescence efficiency of Gd3+ is strongly decreased by Cr2+, Mn2+, and Fe2+ due to energy transfer to these ions. The excitation energy migrates over the Gd3+ ions. Therefore, low concentrations of impurities have a considerable effect on the luminescence efficiency of Gd3+. Since there is no energy migration over the Tb3+ ions, small concentrations of impurities have a limited effect on the luminescence efficiency of Tb3+.