MoSi 2 with additions of 1–4wt.% La 2 O 3 were prepared by spark plasma sintering (SPS). The influence of La 2 O 3 addition on the crystallization and chemical composition of the grain-boundary phase has been studied. It is found that the siliceous grain-boundary phase in MoSi 2 is eliminated completely when the La 2 O 3 /SiO 2 ratio is approximately 3.2 by weight. The siliceous grain-boundary phase reacts with La 2 O 3 addition forming a single crystalline La 2 Si 2 O 7 . In the case of excess addition of La 2 O 3 , there are two polymorphs of lanthanum silicates at trigonal grain boundaries. The main difference of the two polymorphs is the content of elemental La and Si.