Abstract Ga 2Se 3 and Ga 2Se 3:Co 2+ single crystals were produced via the chemical transport reaction method using Ga, Se, and Co as starting materials and (ZnCl 2+I 2) as a transport agent. The single crystals then crystallized into cubic structures with a lattice constant of a=5.442 Å for Ga 2Se 3 and a=5.672 Å for Ga 2Se 3:Co 2+. The optical energy gap of the single crystals was found to be 2.070 eV for Ga 2Se 3 and 1.931 eV for Ga 2Se 3:Co 2+ at 298 K. The temperature dependence of the optical energy gap was fit well with the Varshni equation. Impurity optical absorption appeared in Ga 2Se 3:Co 2+. It was described, by the framework of the crystal field theory, as appearing due to electron transition between the energy levels of the Co 2+ ion sited in T d symmetry.