The formation of solid solution series Cs_1-xM_xGaQ₂-mC64 (M = K, Rb; Q = S, Se; x = 0–1) was studied by X-ray diffraction and spectroscopic methods, revealing a complete miscibility of CsGaQ₂-mC64 with RbGaQ₂ and KGaSe₂, and a large miscibility gap with KGaS₂. All solid solution members exhibit similar Raman spectra, indicating the covalent Ga-Q bonding character. The similar optical band gaps likewise further contribute to this conclusion. Up to a degree of substitution, these solid solutions undergo a phase transition similar to CsGaQ₂-mC64. The influence of the substitution parameter x on phase transition process was investigated in situ using high-temperature X-ray powder diffraction experiments. Phase-pure solid solutions of the high-temperature polymorphs Cs_1-xM_xGaQ₂-mC16 were obtained up to x_max(K) = 0.1 and x_max(Rb) = 0.3. The crystal structures of these new CsGaQ₂-mC16 analogous high-temperature phases were refined from synchrotron diffraction data by Rietveld-refinement.