A hydrothermal reaction between Cu(NO₃)₂ and Na₂WO₄ at 170 °C, followed by sintering at 500 °C, resulted in the formation of stoichiometric CuWO₄ at pH 5.2, while the reactions below and above pH 5.2 gave a mixture of CuWO₄ with WO₃ and CuO, respectively. For organic degradation in water under visible light, W-rich and Cu-rich samples were less and more active than CuWO₄, respectively. A maximum activity was observed with the sample prepared at pH 8.5. Furthermore, this Cu-rich sample shown an activity greatly changing with its sintering temperature, and reaching a maximum at 600 °C. A possible mechanism responsible for the observed activity difference among the samples is proposed, involving the interfacial charge transfer between CuWO₄ and WO₃ or CuO.