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The compounds CdGa2S4 and CdGa2Se4 belong to the defect chalcopyrite (space group 1 ¯4) family. A series of compounds CdGa2S4(1-xSe4x has been prepared and their single crystals grown by the chemical transport reaction method. Optical absorption, spectral response, electrical conductivity and thermoelectric power measurements have been made. The optical absorption spectra revealed that the fundamental absorption edge varies with composition, from 2.2 to 3.25 eV. The plots (ahw)2 versus hw revealed that these compounds are direct band gap materials, but plots of (ahw)1/2 versus hw did not give convincing support to the presence of indirect transitions. The values of energy gaps were also deduced from spectral response (?max) measurements and found to be in agreement with those deduced from optical absorption measurements. D.c. resistivity versus temperature studies revealed that, in spite of their large band gaps, these compounds exhibit intrinsic semiconduction above 250° C. The energy gap values matched with those obtained from other measurements. All samples were n-type and had a constant thermoelectric power ˜ 300 µV° C-1 in the temperature range 250 to 350° C. However, the thermal dependence of electrical conductivity and thermoelectric power indicated strong irreversibility with the thermal heating and cooling cycle. Such behaviour has been attributed to the diffusion of contact materials such Ga and In.