The discovery of lead-based organicinorganic perovskite materials for optoelectronic applications has triggered a revolution in photovoltaic material research. Despite their exceptional material properties such as strong light absorption, long charge carrier lifetimes in combination with high carrier mobility, and low production costs, their long-term instability, and the toxicity of lead currently hamper their deployment at an industrial scale. [1] To overcome this drawback, double perovskites with the general formula A 2 1+ M 1+ M′ 3+ X 6 have been proposed as candidate materials providing leadfree alternatives in the vastly expanding research field of perovskites. One of the first materials investigated in this branch of the perovskite catalog is Cs 2 AgBiBr 6 , demonstrating high stability in devices [2,3] and low effective carrier masses [4] with the long carrier recombination lifetimes Lead-free double perovskites have great potential as stable and nontoxic optoelectronic materials. Recently, Cs 2 AgBiBr 6 has emerged as a promising material, with suboptimal photon-to-charge carrier conversion efficiency, yet well suited for high-energy photon-detection applications. Here, the optoelectronic and structural properties of pure Cs 2 AgBiBr 6 and alkalimetal-substituted (Cs 1−x Y x) 2 AgBiBr 6 (Y: Rb + , K + , Na + ; x = 0.02) single crystals are investigated. Strikingly, alkali-substitution entails a tunability to the material system in its response to X-rays and structural properties that is most strongly revealed in Rb-substituted compounds whose X-ray sensitivity outperforms other double-perovskite-based devices reported. While the fundamental nature and magnitude of the bandgap remains unchanged, the alkali-substituted materials exhibit a threefold boost in their fundamental carrier recombination lifetime at room temperature. Moreover, an enhanced electron-acoustic phonon scattering is found compared to Cs 2 AgBiBr 6. The study thus paves the way for employing cation substitution to tune the properties of double perovskites toward a new material platform for optoelectronics.