Diffusion Brazing (DB) of nickel-based superalloys is a standard joining process widely used in gas turbine industry. The DB technique uses a low melting nickel-based filler material, which contains a rapidly diffusing melting point depressant (MPD) such as boron. Upon brazing, the braze filler firstly melts and then solidifies isothermally through the diffusion of boron from the liquid braze alloy into the solid parent material. Fully isothermally solidified joints exhibit a brittle-phase free microstructure and show excellent mechanical properties. The kinetics of the DB process are controlled by the "effective MPD flux" from the liquid into the surrounding bulk material. The effective MPD flux is a function of the boron diffusion rates and thermodynamic parameters of the parent materials such as the MPD solubility. In the case of brazing superalloys, the complex interactions between the braze filler and the parent material are not fully understood with regard to thermodynamic parameters that control the effective MPD flux. In the present work, the DB process applied to a single crystal multi-component superalloy is studied by combining experimental and numerical modelling results. A representative superalloy/braze filler system has been selected, namely CMSX-4/D-15. The main task consisted of studying the thermodynamic parameters that control the effective MPD (boron) flux into the parent superalloy. Boron diffusion into the superalloy material causes the formation of borides due to the low solubility of the matrix phase for boron and the presence of boride forming elements such as Cr, W and Mo. Boron is therefore steadily consumed from the superalloy matrix phase during isothermal solidification by in-situ boride precipitation. As a consequence, the effective boron flux into the parent material is increased. The overall boron content in the parent superalloy is higher than the boron solubility limit would allow. It has been found that there exists an optimum brazing temperature at which the effective boron flux into the CMSX-4 material due to boride formation is highest. With an increase in the brazing temperature the volume fraction of borides that are stable in the parent material steadily decreases. Above the optimum brazing temperature, the effect of boride formation on the effective boron flux into the superalloy material is low. The DB process is then mainly controlled by the low boron solubility in the matrix phase. The results regarding the effective boron flux and an optimum brazing temperature can be applied to most superalloy systems as most superalloys contain boride forming elements. The present work supports generally the selection of appropriate brazing cycles for the diffusion brazing of superalloys.