The aim of this study was to determine the suitability of a High- Temperature Laser Scanning Microscope (HT- LSCM) for the determination of invariant points. Therefore the first liquid formation of three refractory bricks from service was investigated. Before performing the HT- LSCM measurements, the infiltrated refractory bricks were prepared and the phase paragenesis was determined using a Scanning Electron Microscope and an Energy Dispersive X-Ray Spectroscopy. The infiltrated refractory bricks were then ranked according to the number of phases and the HT LSCM examinations started with the brick with the lowest number of phases. Then the results were compared with phase diagrams and thermochemical calculations. The evaluation of the magnesia brick measurements from a HPS High Performance Shaft Kiln showed a first liquid formation of monticellite in a phase paragenesis from monticellite, merwinite and periclase at a temperature of 1456°C. As expected, this temperature lies just below the invariant point at 1490°C of the CMS- Phase diagram. This temperature difference could be explained by FactSage calculations because the enrichment of Al2O3 lowers the melting point. Measurements of the magnesia spinel brick from a Normal Shaft Kiln showed that on the hot face a liquid formation of MA- spinel and C2S occurred in a temperature range from 1291°C to 1319°C. Just before the infiltration border the first liquid formation of monticellite, merwinite and MA- spinel was observed at 1351°C. These results agree with FactSage calculations. The lowest invariant point was determined at a temperature of 1061°C, directly at the infiltration border and is attributable to the finding of an eutectic of nepheline and kalsilite. The third infiltrated brick, having the highest number of phases, is a magnesia chromite brick from a Top Blown Rotary Converter. The investigation shows an infiltration by Ca- ferritic slag and matte. The infiltration profile showed enrichment on the infiltration border of Cu- Ni- and Fe- sulfides. Due to this, measurements were performed under atmospheric conditions and argon atmosphere in order to observe roasting and melting of the sulfides. Investigations of the non-infiltrated brick, near the infiltration border, showed a first liquid formation of monticellite in a phase paragenesis from monticellite, periclase and chromite at a temperature of 1332°C. Due to the verification by means of phase diagram and FactSage calculations, the High- Temperature Laser Scanning Microscope can be considered as a very suitable device for the investigation of first liquid formation.