Copper smelter slag and copper smelter slag flotation tailings were leached using sulphuric acid solutions, without or with the addition of either ferric sulphate or hydrogen peroxide. Copper extraction from the slag was typically found to be twice as high as that from the slag flotation tailings. Hydrogen peroxide was determined to be the best lixiviant. Thus, copper and iron extractions were 63.4% and 48.6%, respectively, when leaching the slag with 3 M H2O2, after 120 minutes of reaction at room temperature. Copper dissolution was generally found to be the dominant process within the first 60 minutes of reaction, whereas iron dissolution dominated afterwards.