High lime consumption during the neutralisation of oxidised solids at the Lihir gold operation can be related to both ore mineralogy and conditions within the autoclaves. During the pressure oxidation process, basic iron sulphate and potassium jarosite precipitated and silicate clay minerals were dissolved. Illite in the plant feed was identified as a source of potassium for potassium jarosite precipitation - a greater extraction of which was obtained during a period of 'low' lime consumption. By contrast potassium feldspar was found to be largely un-reactive. The dissolution of basic iron sulphate under alkali conditions resulted in the consumption of lime. The difference between the conversion of 4% and 8% of the total solid, by weight, from basic iron sulphate to ferric hydroxide can account for either 'low' or 'high' lime consumption. Potassium jarosite was largely non-reactive during neutralisation. The quantity of basic iron sulphate present in the autoclave discharge is likely to be a function of both ore mineralogy and autoclave conditions. These mineralogical changes occur mostly in the ultrafine, -10 mum size fraction. In order to control the consumption of lime in the plant, it is clear that during pressure oxidation the formation of basic iron sulphates needs to be minimised and that of potassium jarosite promoted.