A SEM-EDS based image analysis method (eMAX) was applied for quantitative determination of the solid composition in pressure oxidation tests of a complex refratory gold ore. The decomposition of sulphides and the formation of secondary phases were calculated on the basis of the image analysis data. The ore examined contained over 60% of its gold as “invisible gold” occurring predominantly in an iron arsenide mineral loellingite (FeAs2), while the other sulphides containe pratically no gold. Therefore, the pressure oxidation test scheme was focused on evaluating the possibilities of selective oxidation of loellingite. eMAX analysis was very useful in determining the amounts of pyrite, pyrrhotite, arsenopyrite, loellingite, and galena as well as the decomposition productes such as ferric arsenate, iron hydroxy sulphate, and hematite. At 180°C, 11 bar oxygen, 96% of the loellingite decomposed in 30 minutes compared with less than 60% of the pyrite and 78% of the total phase. At 210°C the oxidation selectivity between loellingite and other sulphides was poor. The cynidation tests showed that a gold recovery of over 90% was achieved after 30 min autoclaving. The selective oxidation would yield cost savings as reduced oxugen consumption and also result in lower neutralization costs and less low-density waste material.