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The self-diffusion of silicon in single crystal MoSi
2
was studied by means of a radiotracer technique using the short-lived radioisotope
31
Si (half-life
), which was produced and implanted into the samples at the ion-guide isotope separator on-line device at the University of Jyväskylä in Finland. Diffusion annealing and subsequent serial sectioning of the specimens were performed immediately after the radiotracer implantation. In the entire temperature region investigated (835-1124 K), the
31
Si diffusivities in both principal directions of the tetragonal MoSi
2
crystals obey Arrhenius laws, where the diffusion perpendicular to the tetragonal axis is faster than parallel to it. In previous studies the same features were observed for the
71
Ge diffusivities in MoSi
2
, except that these are somewhat higher than those of
31
Si. Furthermore, it is noteworthy that in MoSi
2
the diffusivities of
31
Si and
71
Ge are orders of magnitude faster than the diffusivity of
99
Mo. This large difference suggests that silicon diffusion and molybdenum diffusion are completely decoupled and that silicon diffusion takes place exclusively on the silicon sublattice. Literature data on the phase growth of MoSi
2
are in accordance with the present results on the
31
Si diffusivities; Monte Carlo simulations of the correlation effects of vacancy-mediated diffusion on the silicon sublattice of MoSi
2
lead to their rationalization.