The thiospinel CuIr 2 S 4 exhibits a temperature-induced metal-insulator (M -I) transition around 226 K, showing hysteresis on heating and cooling, that manifests itself as a gap in the electronic density of state with increasing electrical resistivity at low temperatures. Conversely, CuIr 2 Se 4 remains metallic down to 0.5 K. We have successfully synthesized the spinel-type compound CuIr 2 (S 1Ϫx Se x ) 4 system. In order to see the effect of substitutions of Se at the S sites, we have carried out a systematic experimental study of structural, electrical, and magnetic properties of CuIr 2 (S 1Ϫx Se x ) 4 . Mössbauer spectroscopy measurements of 193 Ir have been performed for CuIr 2 S 4 and CuIr 2 Se 4 . The M -I transition of CuIr 2 (S 1Ϫx Se x ) 4 for xр0.15 is accompanied by a structural transformation from tetragonal ͑low-temperature insulating phase͒ to cubic ͑high-temperature metallic phase͒ symmetry. With increasing Se concentration x, the sharp M -I transition shifts to lower temperature. The resistivity shows a monotonous increase with decreasing temperature for 0.17рxр0.78 between 4.2 and 300 K, and the metallic state is recovered for xу0.80. Magnetic susceptibility measurements show the jump at the M -I transition temperature with hysteresis on heating and cooling. The high-temperature metallic phase of CuIr 2 S 4 shows Pauli paramagnetism, having a density of states at the Fermi level, D(⑀ F )ϭ0.67 states/eV atom. The insulating phase at low temperatures exhibits diamagnetism, and there is no localized magnetic moment. The Arrhenius regime is observed for the conductivity with a thermally activated process for 0рxр0.70 in the insulating phase. There is a general trend toward increasing metallicity with increasing x, which is consistent with the magnetic susceptibility results. A possibility of a two-site model of different valence states for Ir ions in the insulating phase of CuIr 2 S 4 will be discussed on the basis of the Mössbauer data. A phase diagram of temperature versus Se concentration x will be proposed for the CuIr 2 (S 1Ϫx Se x ) 4 system. The mechanism of the M -I transition remains enigmatic and is far from a complete picture. ͓S0163-1829͑98͒01535-5͔