The four new quaternary copper (I)-selenides, Sr 0.5 CuZrSe 3 (a= 3.8386 (7), b= 14.197 (2), c= 10.1577 (17) Å), Ba 0.5 CuZrSe 3 (a= 3.8386 (7), b= 14.196 (2), c= 10.1577 (17) Å), SrCuYSe 3 (a= 10.620 (2), b= 4.1000 (8), c= 13.540 (3) Å) and BaCuYSe 3 (a= 4.1800 (7), b= 13.940 (2), c= 10.6200 (17) Å) were synthesized by high-temperature solid state reactions and their crystal structures were determined using single-crystal X-ray diffraction. A 0.5 CuZrSe 3 (A= Sr, Ba) and BaCuYSe 3 crystallize in the KCuZrS 3 structure type (Cmcm), while SrCuYSe 3 is isostructural to Eu 2 CuS 3 (Pnma). All compounds form layered structures in which the charge of the [CuZrSe 3]−∞ 2 and [CuYSe 3] 2−∞ 2 layers as well as the site occupancy of the A cations depend on the transition metal. Combining the alkaline earth metals Sr and Ba with tetravalent Zr leads to the formation of cation vacancies between the [CuZrSe 3]−∞ 2 layers and structure type as well as symmetry are determined by the ratio between the cation and transition metal ionic radii r (A 2+)/r (M 3+/4+).