The aim of this diploma thesis was to examine the oxidation state of vanadium containing slags produced at the FeV45 production at the Treibacher Industrie AG in order to find a way to decrease the valenz state of vanadium in various complex compounds to minimize the demand of the reduction metal aluminium in the following reduction step of the ferrovanadium production. Using the thermodynamic calculation program FactSage a forecast was made to get an idea, which conditions and parameters influence the behaviour of the metal/slag equilibrium mostly. Another chapter of this diploma thesis is occupied with the prereduction of vanadium containing slags in a rotary kiln using hydrogen as reduction gas. These examinations were made on an existing laboratory rotary kiln at the Institute of nonferrous metallurgy. In addition to this RFA, RDA, EDX and Rietveld analysis of various slag types were made to check the forecast of the simulations made with FactSage. The third main point of this thesis included the influence of the transportation of phosphor into the slag, which prevents the selective oxidation of the remaining vanadium in the melt.