Phosphate hosts doped with luminescent ions are widely applied for efficient spectral conversion in light emitting devices. However, in Xe discharge lamps a rapid degradation of light output is observed indicating a decrease in the efficiency of conversion of the VUV output of the Xe discharge. This work aims at providing a better understanding of the degradation mechanism. We summarize peculiar findings made within a survey of different inorganic UV emitting phosphors of the MPO4:X (M = Y, La, Lu; X = Bi, Pr, Gd) type regarding their stability under operation in a dielectric barrier Xe excimer discharge lamp. Test lamps containing these phosphors as a particle layer coating onto the inner wall of the lamp vessel suffer from a rapid degradation which was monitored by the loss of UV radiation output during lamp operation. The thus aged materials were recovered and further analysed by optical spectroscopy. It turned out that the aged materials do not only exhibit a new broad absorption in the UV/visible range, which explains the rapid decline of radiation output during lamp operation, but also that all aged materials show photoluminescence in the red upon exposure to UV radiation. It is argued that this luminescence originates from phosphorus in the oxidation state +III (configuration: [Ne]3 s2) which is present as a point defect. These defects are formed when the respective materials are aged in direct contact with the Xe excimer discharge within the lamp discharge vessel. This detailed study of phosphor aging by means of VUV luminescence spectroscopy as well as temperature and time dependent luminescence spectroscopy and solid state EPR spectroscopy points to the existence of phosphorus +III ([Ne]3 s2) which shows characteristic ns2 luminescence which has not been reported before.