Detailed first-principles calculations of the structural, electronic, and optical properties of solid solutions of the promising solar cell material CuAl(S1-xSex)(2) over the whole range of Se concentration from x = 0 to x = 1 were performed. It was established that the calculated lattice parameters, band gap, and anisotropic refractive indices vary linearly with the Se concentration. The obtained linear dependences allow for reliable estimations of all these quantities for any value of x, which determines the solid solution composition. The calculated results were compared with the experimental data available for x = 0, 0.5, and 1.0; very good agreement was demonstrated, which gives confidence in the properties calculated for other Se concentrations (x = 0.25, 0.75). The findings from the present paper can be used in a straightforward way for the successful production of CuAl(S1-xSex)(2) mixed compounds with desired optoelectronic parameters, which are defined by the composition-tuned mobility of the charge carriers in the upper valence band and the conduction band. Extension of the presented approach to other materials is also possible.