Chalcopyrite quaternary semiconductor CuIn(SexS1-x)2 nanocrystallites (0 ≤ x ≤ 1) were successfully prepared via a relatively simple and convenient solvothermal route. From X-ray data it is found that the cell constants a and c vary linearly with the composition x as: a(Å) = 5.5299 + 0.2665x, c(Å) = 11.1004 + 0.5139x. The sample CuInSeS was characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and infrared spectroscopy (IR). The optical property of CuIn(SexS1-x)2 nanocrystallites was also recorded by means of UV-Vis absorption spectroscopy. In addition, the effect of different solvents on the formation of the product was studied to indicate that ethylenediamine is the optimal solvent for this reaction. Finally, the reaction mechanism in ethylenediamine was also investigated.