The results of a study of the inclusions in primary aluminium are presented. Beside complex inclusions, where components with Ca are soluble in water, the research of primary aluminium also revealed the eutectic-like inclusions with 21,5 at. % molybdenum. Careful chemical analysing of trace elements revealed that molybdenum was already present in primary aluminium, so we suppose the most probable source of molybdenum is alumina. Despite efforts, no data could be obtained from alumina producers about the molybdenum content in alumina.