The oxidation and acid production behavior of both ore and coal pyrites with and without a hydrophobic surface coating of sodium oleate was investigated. The results showed that pyrite surface was passivated by sodium oleate coating, which substantially reduced or even eliminated moist air oxidation. Without the hydrophobic surface coating, coal pyrite displayed higher oxidation and acid production rate than ore pyrite. It was also found that the acid production by pyrite in aqueous solution obeys a zeroth order dissolution reaction with respect to hydrogen ions. The nature of the surface coating was characterized by electrochemical, contact angle and zeta potential measurements. (C) 2000 Elsevier Science B.V. All rights reserved.