For two types of steel, the heat-resistant 12% Cr-steel X 20 CrMoV 12 1 and the fine-grained pressure vessel steel 20 MnMoNi 5 5 (A 508 cl. 3), a wide range of toughness values in the upper shelf was realized by different sulfur contents and the inclusion of similar weld metals. Additionally, the pressure vessel steel 22 NiMoCr 3 7 (A 508 cl. 2) was also investigated. Tensile, fracture mechanics and Charpy V-notch impact tests, as well as detailed microstructural investigations with respect to the size distribution and density of nonmetallic inclusions and precipitates, were carried out. In order to ensure ductile behaviour, a test temperature of 150 degrees C was chosen. The relevance of two quantitative relations available for the calculation of the J-integral, J(i,phys), at physical crack initiation using tensile test data and microstructural parameters, were examined by comparison with the corresponding experimental J(i,phys)-values. Only one quantitative relation was able to give agreement between calculated and experimental J(i,phys)-values. This holds not only for the base materials but also for the weld metals. The importance of the size and density of the non-metallic inclusions became quantitatively obvious with the consequence that their size times density is a decisive parameter for toughness. Observations of void initiation, growth and coalescence illustrate the fracture process.