Phosphors suffer from decreasing light output due to degradation on exposure to heat, oxygen and water during the device fabrication process and in the working environment. Here, we report a series of (Ba,Sr)LuAl2Si2O2N5:Ce3+,Eu2+ phosphors which retain their initial luminescence intensity after being annealed in air at up to 800 °C or immersed in water for 5 days. Powder X-ray diffraction for fresh and annealed samples and thermogravimetric/differential thermal analysis confirmed the high stability of the host lattice of (Ba,Sr)LuAl2Si2O2N5. Electron spin resonance showed that the majority of Eu remained in the divalent state even after heating at 800 °C in air. The robust host lattice and stable valence of the activators are responsible for the high stability of (Ba,Sr)LuAl2Si2O2N5:Ce3+,Eu2+ phosphors. The high chemical stability originates from the featured star-like N[(Al/Si)(O/N)3]4 building block and the condensed three dimensional rigid framework. The (Ba,Sr)LuAl2Si2O2N5:Ce3+,Eu2+ phosphors are potential candidates for lighting and display applications, especially for those that need high temperature treatment in the packaging process.