The conditions of phase formation in the ternary water-salt system comprised of NiSO4–CoSO4–H2O at 25 °C, which is the first step for the creation of structurally perfect mixed crystals CoxNi(2–x)(SO4)2·12H2O, have been studied. The crystallization regions of tetragonal NiSO4·6H2O in the NiSO4:CoSO4 ratio range from 100:0 to 95:5 mass% and monoclinic solid solution of CoxNi(2–x)(SO4)2·12H2O in the NiSO4:CoSO4 ratio range from 95:5 to 0:100 mass% were delineated. A set of CoxNi(2–x)(SO4)2·12H2O single crystals in the range of 0.05 ≤ x ≤ 1.95 was obtained. The crystal structures of the samples with x values equal to 1.5, 1.13, and 0.56 have been studied. The CoxNi(2–x)(SO4)2·12H2O crystals are a series of solid solutions based on the crystal structure of CoSO4·6H2O. The transmission spectra in the λ range of 200–800 nm were studied for the obtained crystal samples. A significant decrease in the intensity of the transmission bands in the visible region of the spectrum was observed due to the simultaneous presence of [Ni(H2O)6] and [Co(H2O)6] complex octahedra in the crystal structures. Consequently, the CoxNi(2–x)(SO4)2·12H2O crystals are a promising material for optical filters used in photonics.