The extent of galena, sphalerite and pyrite oxidation may be determined by leaching slurries with an EDTA solution and analysing the filtrate for cations. EDTA will selectively dissolve the oxidised metals but not their sulphides. The extraction must be performed in the absence of oxygen and at an alkaline pH. Sufficient time should be allowed for complete dissolution and there should be adequate EDTA to dissolve all metals which form more stable complexes with EDTA than the metal of interest. The technique indicates that Hellyer ore is fresh before milling, mildly oxidised during milling and extensively oxidised during flotation. The technique has also shown that in freshly ground Hellyer ore, galena oxidation is rapid and extensive; pyrite oxidation is rapid but limited due to the onset of passivation while sphalerite oxidation is slow.