The quaternary alkali-metal gallium selenostannates, Na 2−x Ga 2−x Sn 1+x Se 6 and AGaSnSe 4 (A=K, Rb, and Cs), were synthesized by reacting alkali-metal selenide, Ga, Sn, and Se with a flame melting–rapid cooling method. Na 2−x Ga 2−x Sn 1+x Se 6 crystallizes in the non-centrosymmetric space group C2 with cell constants a=13.308(3)Å, b=7.594(2)Å, c=13.842(3)Å, β=118.730(4)°, V=1226.7(5)Å 3 . α-KGaSnSe 4 crystallizes in the tetragonal space group I4/mcm with a=8.186(5)Å and c=6.403(5)Å, V=429.1(5)Å 3 . β-KGaSnSe 4 crystallizes in the space group P2 1 /c with cell constants a=7.490(2)Å, b=12.578(3)Å, c=18.306(5)Å, β=98.653(5)°, V=1705.0(8)Å 3 . The unit cell of isostructural RbGaSnSe 4 is a=7.567(2)Å, b=12.656(3)Å, c=18.277(4)Å, β=95.924(4)°, V=1741.1(7)Å 3 . CsGaSnSe 4 crystallizes in the orthorhombic space group Pmcn with a=7.679(2)Å, b=12.655(3)Å, c=18.278(5)Å, V=1776.1(8)Å 3 . The structure of Na 2−x Ga 2−x Sn 1+x Se 6 consists of a polar three-dimensional network of trimeric (Sn,Ga) 3 Se 9 units with Na atoms located in tunnels. The AGaSnSe 4 possess layered structures. The compounds show nearly the same Raman spectral features, except for Na 2−x Ga 2−x Sn 1+x Se 6 . Optical band gaps, determined from UV–Vis spectroscopy, range from 1.50eV in Na 2−x Ga 2−x Sn 1+x Se 6 to 1.97eV in CsGaSnSe 4 . Cooling of the melts of KGaSnSe 4 and RbGaSnSe 4 produces only kinetically stable products. The thermodynamically stable product is accessible under extended annealing, which leads to the so-called γ-form (BaGa 2 S 4 -type) of these compounds.