The system CoIn2S4xSe4(1-x) has been investigated by X-ray powder methods on samples quenched at 700degreesC. The spinel type phase has a phase width of 1 greater than or equal to x greater than or equal to 0.9. A new layered compound is formed for 0.9 greater than or equal to x greater than or equal to 0.45 which crystallizes with the alpha-FeGa2S4-type with a = 392.6 pm and c = 1270.3 pm (x = 0.5) for the hexagonal cell. Platelike crystals of the layered phase are obtained by transport reactions with iodine in a temperature gradient 750 --> 700degreesC. The band gaps of these crystals measured by optical absorption vary from 1.2 to 1.4 eV. The electrical conductivities of the crystals are found in the order of 10(-5) Omega(-1) cm(-1). (C) 2003 Elsevier Science Ltd. All rights reserved.