Abstract The degree of oxidation during grinding and its effect on galena flotation were studied using a specialised mill which permitted chemical conditions during grinding to be controlled. It was found that iron oxidation species derived from the grinding medium played a dominant role in galena flotation. The amount of hydroxide consumed to maintain the grinding pH at a constant value depended on the production of iron oxidation species. Linear relationships were obtained between the amount of hydroxide consumed to maintain a constant grinding pH and the production of iron oxidation species as well as galena recovery. These relationships should eventually permit oxidation during grinding and galena flotation to be predicted.