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Novel composite photocatalysts consisting of a cadmium and zinc sulfide solid solution (Cd 0.6 Zn 0.4 S) and zinc sulfide (ZnS) nanoparticles were successfully prepared by a simple hydrothermal treatment of suspended Cd 0.3 Zn 0.7 S 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 Na 2 S/Na 2 SO 3 aqueous solution under visible light irradiation (λ = 450 nm). It is shown that the hydrothermal treatment of Cd 0.3 Zn 0.7 S 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 Cd 0.6 Zn 0.4 S/ZnS sample. The deposition of gold on the surface of Cd 0.6 Zn 0.4 S/ZnS leads to a further increase in activity: the achieved photocatalytic activity and quantum efficiency (450 nm) for 1%Au/Cd 0.6 Zn 0.4 S/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 Cd 1-x Zn x S from the cubic to the hexagonal phase during the hydrothermal treatment. Additionally, photoelectrodes based on Cd 0.6 Zn 0.4 S/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 Cd 0.6 Zn 0.4 S/ZnS/FTO electrode. An investigation by means of impedance spectroscopy reveals the longer lifetime of photogenerated charge carriers in the Cd 0.6 Zn 0.4 S/ZnS/FTO photoelectrode if to compare with Cd 0.3 Zn 0.7 S/FTO system. Image 1 • Au/Cd 0.6 Zn 0.4 S/ZnS photocatalysts were prepared via simple hydrothermal synthesis. • The highest value of the photocatalytic activity was 17.4 mmol H 2 per hour per gram. • The highest apparent quantum efficiency was 42.6% (450 nm). • Photocurrent 0.5 mA/cm2 was achieved for the Cd 0.6 Zn 0.4 S/ZnS/FTO electrode. [ABSTRACT FROM AUTHOR]