Thermal vacancy formation was studied in MoSi(2) by the temperature dependence of the mean positron life time and of the W parameter. A low vacancy formation enthalpy of H(V)(F)=(1.6 +/- 0.1) eV was determined and a low migration enthalpy H(V)(M) was estimated. Coincident measurement of the Doppler broadening of the positron-electron annihilation radiation at high electron momenta indicate thermal vacancy formation predominantly on the Si-sublattice. A high thermal vacancy concentration and a high vacancy mobility on the Si-sublattice explains directly the observation that Si diffusion in MoSi(2) is substantially faster than Mo diffusion.