Optical properties of the spinel type crystals doped with transition metal ions have been studied elsewhere mainly with oxide compounds due to their potentially large area of implementation as active media for solidstate lasers and to a variety of interesting electrical, magnetic, and optical properties. Currently the investigations of the transition metal doped sulphide spinel compounds are in the focus of attention of the author of this paper. The purpose of this work is the study of the radiative properties of the Co-doped α-ZnAl2S4 wide bandgap semiconductor. Bulk Co-doped α-ZnAl2S4 crystals with impurity concentration 0.1-0.5 at.% were grown by a closed tube vapor method with halogen as a transport agent. The X-ray diffraction measurements confirmed that the ZnAl2S4: Co samples were crystallized in the spinel structure with Oh (Fd3m) symmetry. The absorption spectra measured at room temperature in the energy range 0.62 – 4 eV consist of at least four bands centered at 2.59, 1.75, 1.68 and 1.61 eV, the last one being the most prominent. The excitation of the photoluminescence (PL) was provided by an OPO system at three wavelengths: λex1=496nm, λex2=533nm, and λex2=670nm (in the region of the absorption spectral bands). The PL measurements were carried out in the temperature range T=10-300K. The observed absorption and emission spectra (Fig.1) are assigned to electron transitions in the tetrahedral complex consisting of a Co2+-ion and four ligands of the sulfur atoms. The Co2+ion has the d7 configuration and the energy level structure of the Co2+ ion in a tetrahedral site is similar to the level structure of d3 ions in octahedral sites. According to the TanabeSugano diagrams, the bands at the 1.75, 1.68 and 1.61 eV detected in absorption spectrum, that are placed almost in the same spectral region as in other sulfurous compounds doped with Co2+[1-2], are assigned to the transition from the ground state 4A2g(4F) to the 4T1g(4P) excited level, triple degenerated by the spin-orbit coupling. In the case of Td symmetry, the intensity of the crystalline field is weaker than in the octahedral - Oh symmetry; and by analyzing the absorption and luminescent spectra it has been found out that the intensity of the crystalline field of the compound α-ZnAl2S4: Co2+ is weaker than that of 1Dq/B. In this region of the Tanabe-Sugano diagrams for d7 elements in Td symmetry, the two excited levels 4T1g(4P) and 4T2g(4F) are almost parallel, which means the optical transitions between them are always represented by narrow spectral lines in optical spectra. Therefore, the set of the narrow lines observed in the luminescence spectrum at low temperature could be assigned to the transitions from the 4T1g(4P) excited level to the lower 4T2g(4F) that is also degenerated by spin-orbit coupling. A configuration coordinate diagram is suggested to explain the temperature behavior of the observed PL spectra under different wavelengths of optical excitation.figureFig.1 Absorption and emission spectra of the α -ZnAl2S4:Co