Molybdenum is a high-strength refractory metal with outstanding physical and chemical properties. However, its application can be limited by the characteristics of the grain boundaries and their influence on strength and ductility. The fracture mode is significantly influenced by elements at the grain boundaries. It is assumed that alloying elements like boron and carbon segregate to the boundaries and may strengthen them. Due to the small amount of impurities in molybdenum processed nowadays, high resolution techniques, like atom probe tomography and transmission electron microscopy, are necessary. Therefore, site specific preparation methods are required to study the grain boundary chemistry effectively. With the novel method of transmission Kikuchi diffraction a grain boundary can easily be positioned in the first 200 nm of an atom probe tip. Furthermore, this high-resolution technique gives the opportunity to get crystallographic information of the mapped area and, therefore, an analysis of the grain boundary character. In the present study grain boundaries of carbon and boron doped sintered molybdenum alloys are compared with technically pure molybdenum by atom probe tomography in sintered condition. Additionally, the microstructure is analysed by electron backscatter diffraction and transmission electron microscopy, as well as the mechanical properties with 3-point-bending tests and tensile tests. Due to the addition of carbon and boron, a change in the fracture mode occurred. Furthermore, a decrease of the ductile-brittle transition to lower temperature is observed. These segregation at the grain boundaries are detected and analysed by atom probe tomography. Consequently, the atomic grain boundary chemistry can be correlated with the macroscopic strength values. The difference of segregation content at the grain boundaries is discussed.