Abstract The structural and optical properties of NaxCu1−xIn5S8 powders, with x = 0 , 0.25, 0.5, 0.75 and 1, are determined. It is shown that all of the samples have the same crystalline structure, which indicates the existence of a solid solution over the whole range of x (i.e. 0 ⩽ x ⩽ 1 ). The increase of the optical band gap of these compounds is found to be linear between x = 0 ( E g = 1.5 0 eV ) and x = 0.7 5 (1.85 eV), whereas for x = 1 (i.e. NaIn5S8) Eg is found to be 2.40 eV, which is much higher than the value expected from the linear slope. Such an evolution shows that the copper and the sodium weigh differently on the optical properties of the material. In thin film solar cells with indium sulfide buffer layer, NaxCu1−xIn5S8 compounds are formed at the Cu(In,Ga)Se2/In2S3 interface. The impact of their properties, here determined, on these thin film solar cells is discussed.