Potassium doped tungsten (wolfram-vacuum-metallizing - WVM) subjected to intense deformation results in a material with an elongated grain structure after recrystallization and high creep resistance. Due to the high recrystallization temperature of WVM it is used to form wires for applications such as filaments in lamps for the lighting industry and stationary anodes for medical-radiography purposes. The production route for WVM consists of steps of rolling and drawing. In this thesis a study has been undertaken to investigate the longitudinal splitting of WVM-wires that may occur upon rolling or subsequent processing. Fracture toughness has been investigated by means of 3-point bending specimens and double cantilever beam specimens. Tests were performed in the range room temperature to more than 1000°C. Electron backscatter diffraction, auger electron spectroscopy and x-ray line profile analysis was used to determine the influence of texture, chemistry and dislocation density on fracture toughness. New testing techniques have been devised and successfully applied to determine the fracture toughness of WVM especially in the longitudinal direction at high temperatures.