Several researchers have shown that the surface of chalcopyrite becomes hydrophobic under oxidizing conditions. Because dissolution of copper from chalcopyrite in dump leaching occurs under oxidizing conditions, the U.S. Bureau of Mines studied the effect of surfactants on chalcopyrite leaching under dump chemical conditions. Surfactants were selected for study because they can lower interfacial tension. The tests were conducted in shaking water baths using 250-ml erlenmeyer flasks in statistical experimental design formats. The addition of nonionic surfactants increased copper extraction in the ferric sulfate leaching of chalcopyrite when sufficient fe3+ was present. A positive interaction between fe3+ concentration and surfactant concentration was demonstrated. Copper concentration in the leaching solutions was increased by as much as 100 pct at 25 deg and 50 deg c by adding nonionic surfactants. Ionic surfactants decreased leaching. The structure of the nonionic surfactants and their hydrophile-lipophile balance (hlb) influenced surfactant performance. The branched hydrophile of an ethoxypolyol was beneficial to surfactant performance. The ethoxypolyol was the superior surfactant at 25 deg c, but became unstable at 50 deg c. Four surfactants, an ethoxyalchohol and three block copolymers, performed best at 50 deg c.