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Sodium hydrogen sulfide (NaHS) is commonly used as a copper depressant in the selective flotation of copper and molybdenum ores. However, the process is facing health and safety issues because NaHS readily yields toxic hydrogen sulfide gas (H 2 S) under acidic conditions. In this study, Na 2 SO 3 was proposed as an alternative copper depressant. The effect of Na 2 SO 3 on the surface wettability and floatability of chalcopyrite and molybdenite-typical copper and molybdenum minerals, respectively-was intensively studied using contact angle measurements and flotation tests. Contact angle readings show that the chalcopyrite surface became hydrophilic after the Na 2 SO 3 treatment. Meanwhile, the molybdenite surface was relatively more hydrophobic compared with that of chalcopyrite after the treatment. Flotation tests using pure minerals of chalcopyrite and molybdenite demonstrate that the floatability of chalcopyrite decreased with increasing concentration of Na 2 SO 3. On the other hand, the floatability of molybdenite gradually increased under similar conditions, suggesting that Na 2 SO 3 might have the potential to be used for selective flotation of chalcopyrite and molybdenite. A possible mechanism is proposed in this study to explain the phenomenon using X-ray photoelectron spectroscopy analysis.