In the leaching of non-ferrous smelter slag with a dissolved gas mixture of SO2 and O-2 the behaviour of Co, Cu, Ni, Zn and Fe was studied in a 1-L. batch reactor. The parameters investigated include P-SO2, P-O2' temperature, particle size and pH. Co, Zn and Fe behaved similarly while Ni and Cu displayed distinguishable characters. The addition of O-2 prevented the precipitation of Cu after dissolution, and increased acidity of the leaching solution. The increase in acid strength resulted in an Increase in,the extraction of Co, Zn and Fe. The effect of acidity on Cu and Ni extraction was however much weaker. The combination of SO2 and O-2 was found to be a move effective oxidant of Fe(II) to Fe(III) than O-2 alone. Simultaneous extraction of valuable metals, and removal of Fe could be achieved by leaching at pH of 3 to 4. Maximum selectivities obtained for Co, Ni and Cu over Fe were 300, 2000 and 4000, respectively.