Assessments of diffusion mobility parameters are performed for diffusion in the bcc Mo-Cr-Fe system by taking available literature data into account. The Mo mobility is in main focus where, in addition to all binaries, ternary interaction parameters are assessed. A diffusion couple is produced and heat treated to cause elemental interdiffusion which is measured experimentally and compared with DICTRA calculations. The obtained experimental information is used for optimization of a mobility parameter. An experiment is also performed in order to study the coarsening of Mo rich M6C carbides embedded in a ferrite matrix, where the diffusion of Mo is believed to be of major importance. The measured coarsening rate is compared with the, by DICTRA, calculated rate showing improved agreement when the assessed kinetic description is applied. We judge the assessment of the bcc Mo- Fe-Cr system to be an important step towards a confident use of kinetic calculations as a tool for development of tool steel materials where Mo diffusion is of importance; for instance, when calculating secondary carbide precipitation during tempering of tool steels.