Eu3+-doped alkali metal double molybdate and tungstate phosphors, (MEu) (Mo,WO4)(2) (MEM and MEW, M = Li+, Na+, and K+), have been identified as potential candidate materials for white light-emitting diodes. However, there are no systematic considerations of the role of auxiliary alkali-metal ions (AMIs) on the structural, chemical, and luminescent properties of these phosphors. Here, a systematic investigation to elucidate the role of AMIs on MEM and MEW phosphors was performed using ab initio calculations based on experimental evidence. The results from experimental assessments with intensive structural and chemical bond analyses reveal a clear dependence of AMIs on the structural and luminescent properties of phosphors. The structural changes due to AMIs originate from a bond length change between AMIs-O2- bonds, and AMIs had minor effects on the other bonds. A phase transition induced by K+ ions originates from not only structural factors but also local chemical bonding characteristics. Overall, the results indicate that the major role of AMIs is to induce phosphor structural changes, and the effects on the chemical properties are minor. The results of this study shed light on developing not only MEM and MEW phosphors but also new phosphor materials using auxiliary metal ions.