99Mo tracer diffusion in Fe79-yMo8Cu1B12+y (y = 0, 3, 5) alloys was studied using serial sectioning method in the temperature range 573 – 773 K. Sputtering by Ar+ ions was applied as a sectioning technique. The measurement was carried out with the ribbon-like samples prepared by planar flow casting technique and crystallized before the diffusion experiment by a thermal shock in vacuum. The structure was formed by areas of three distinct types differing in chemical composition r = at. % Mo / at. % Fe: (i) r = 0.02 (crystallites of Fe, mean size d ~ 150 nm), (ii) r = 0.09 (relaxed phase; d ~ 150 nm) and (iii) r = 0.83 (boride phase; d ~ 30 nm). The measured profiles were linear in co-ordinates ln cMo vs. x1.5 (cMo – concentration of Mo in depth x), indicating that the diffusion of 99Mo was controlled by „type A-B” kinetics.