O-CaO glasses developed for the immobilization of Mo-rich nuclear wastes. Increasing amounts of B 2 O 3 and MoO 3 were added to a SiO 2 -Na 2 O-CaO glass, and crystallization tendency was studied. Na 2 MoO 4 crystallization tendency decreased with the increase of B 2 O 3 concentration whereas the tendency of CaMoO 4 to crystallize increased due to preferential charge compensation of BO 4 À entities by Na 1 ions. 29 Si MAS NMR showed that molybdenum acts as a reticulating agent in glass structure. Trivalent actinides surrogate (Nd 31 ) were shown to enter into CaMoO 4 crystals formed in glasses.