This work is a part of a COST programme undertaken to study the influence of some impurities on the properties of cemented carbides. The precipitation of inclusions containing Ca, Al, Si, P, S and O was studied by combining calculated phase equilibria with sintering experiments based on two cemented carbide grades. Minor amounts of Ca, Al, P and Si were added to the raw materials used and by sintering in furnace atmospheres, having increased sulphur or oxygen potentials, the formation of oxides and sulphides was facilitated. A comprehensive investigation of as-sintered surfaces, polished cross sections and fracture surfaces was made, and the type and amount of impurity-containing phases were compared to the calculated diagrams. It was found that CaS is always formed whenever Ca and S are present. Excess amounts of either Ca or S were evaporated, and also Si to some extent. Aluminum is often found in oxides together with Ca and Si. Both Al and P are present in the sintered material to the same extent as had been added to the raw materials. No phosphorus or very small amounts of the phosphorus added were found in the inclusions investigated. It is believed that P is evenly distributed throughout the material to a high degree.