The results of investigations during the thermal treatment of low-grade high-silicon and high-iron boehmite bauxite from Western Serbia are reported. The results indicate that the basic minerals constituting the investigated bauxite are mutually isomorphous, so that the eventual enrichment of these bauxites by separation of the basic minerals (boehmite, kaolinite and haematite) is only possible by exploiting differences in their chemical properties. Physical methods of separation are not helpful because the initial isomorphism of the minerals is retained during thermal decomposition.