Novel composite photocatalysts consisting of a cadmium and zinc sulfide solid solution (Cd0.6Zn0.4S) and zinc sulfide (ZnS) nanoparticles were successfully prepared by a simple hydrothermal treatment of suspended Cd0.3Zn0.7S at 120 °C. The as-obtained materials were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and UV-VIS diffuse reflection spectroscopy. The obtained photocatalysts were tested in the photocatalytic evolution of hydrogen from a Na2S/Na2SO3 aqueous solution under visible light irradiation (λ = 450 nm). It is shown that the hydrothermal treatment of Cd0.3Zn0.7S at 120 °C increases the activity by a factor of 7.5 due to the phase transformations of the solid solution with the formation of the multiphase Cd0.6Zn0.4S/ZnS sample. The deposition of gold on the surface of Cd0.6Zn0.4S/ZnS leads to a further increase in activity: the achieved photocatalytic activity and quantum efficiency (450 nm) for 1%Au/Cd0.6Zn0.4S/ZnS are 17.4 mmol g−1 h−1 and 42.6%, respectively. This excellent performance is found to be attributable to the transformation of Cd1-xZnxS from the cubic to the hexagonal phase during the hydrothermal treatment. Additionally, photoelectrodes based on Cd0.6Zn0.4S/ZnS and FTO were synthesized and tested in a two-electrode cell. A high value of the photocurrent equal to 0.5 mA/cm2 is achieved for the Cd0.6Zn0.4S/ZnS/FTO electrode. An investigation by means of impedance spectroscopy reveals the longer lifetime of photogenerated charge carriers in the Cd0.6Zn0.4S/ZnS/FTO photoelectrode if to compare with Cd0.3Zn0.7S/FTO system.