Interstitial impurities in sintered Mo La 2O 3 bars have been investigated systematically using SEM, EDAX, XRD and AES. It has been shown that the bend strength and ductility of the sintered Mo bars are greatly improved by the addition of La 2O 3. Interstitial impurities not only exist in the Mo grains, but also segregate to the surface of the La 2O 3 particles and in so doing decrease impurity concentration of the grain boundary.