Welding properties (position welding etc.) of FCAW-consumables strongly depend on thermophysical properties of the slags. The aim of this work, was to characterize three different FCAW-consumables with respect to their thermo-physical properties. These electrodes are, depending on composition of the slags, used for horizontal and position welding of high alloyed steels and nickel based alloys. In the course of this work, investigations by SEM/EDX, light microscopy and Differential Thermal Analysis (DTA) were carried out. Moreover an investigation in HT-LSCM and a viscosity measurement was done. Basic correlations between therm-physical properties of the slags and their functionality could be identified. Stability ranges of several phases are of detrimental importance for position welding. A qualitative explanation of the operation of the slags could be derived from the obtained knowledge. Moreover a possible connection between slag basicity, oxygen content of the weld deposit and hot cracking susceptibility of NIBAS 625 PW-FD-FCAW-consumables was investigated. For this purpose, five wires with different slag basicity were investigated regarding chemical analysis, hot cracking susceptibility and welding behaviour. The quantification of hot cracking susceptibility was carried out by T-joint weld cracking test and Programmable Deformation Rate (PVR) test. The supposed connection could not be clearly verified on basis of the results.