AbstractWith increasing interest in mixed‐anionic hydrides, a number of interesting properties have been reported. Here, the structural and optical properties of (Eu2+‐doped) MCaH3–MCaF3 (M = Rb, Cs) are investigated. For M = Rb, a complete hydride–fluoride solid solution series is found and for M = Cs, the known solid solution series (0 ≤ x ≤ 1.70) can be extended to x = 3. In case of Cs, a very bright luminescence emission is observed in Eu2+‐doped samples, whereas the luminescence is fairly weak in Rb based compounds. With increasing hydride content, a shift of the emission color from cyan‐green to red can be observed. In contrast to earlier reports for mixed fluoride–hydride host, the redshift is not a gradual shift of a single broad emission band, but the appearance of new narrow emission bands on the low energy side, which are assigned to the occupation of sites with higher hydride content. Consequently, this finding represents the first example in a mixed anionic hydride with a site‐sensitive emission for sites with locally varying hydride content in the first coordination sphere and may serve as a general example for emission color tuning taking advantage of mixed‐anionic compounds.