In the vitrification of HLW the aim is to create a product, the stability (thermal, mechanical) of which is, to the greatest possible degree, independent of fluctuations in its composition. Process control during vitrification of HLLW is necessary in order to assure the quality of the final glass product. The chemical processes are controlled by quantitative analysis of all material streams entering and leaving the melting device. There are six material streams to consider: frit input, waste input, corrosion products (generated by the vitrification process), off-gas, recycled off-gas, glass output. Additionally, the capacity of the glass for buffering short-term fluctuations in the incoming streams must be known. Sufficient sampling during non-radioactive vitrification in addition to materials tests can provide the necessary data basis. The results are combined in a materials balance.