Reactivity in the solid state between CoWO 4 and some rare-earth metal tungstates RE 2 WO 6 (RE=Sm, Eu, Gd) was investigated by the XRD method. Two families of new isostructural cobalt and rare-earth metal tungstates, Co 2 RE 2 W 3 O 14 and CoRE 4 W 3 O 16 , were synthesized. The Co 2 RE 2 W 3 O 14 phases are formed by heating in air the CoWO 4 and RE 2 WO 6 compounds mixed at the molar ratio 2:1, while the CoRE 4 W 3 O 16 phases are synthesized at the molar ratio of CoWO 4 /RE 2 WO 6 equals to 1:2. The Co 2 RE 2 W 3 O 14 phases as well as the CoRE 4 W 3 O 16 compounds crystallize in the orthorhombic system. The Co 2 RE 2 W 3 O 14 and CoRE 4 W 3 O 16 compound melt above 1150°C. A melting manner of the Co 2 RE 2 W 3 O 14 and CoRE 4 W 3 O 16 compounds was determined in an inert atmosphere. The formation of CoWO 4−x phase was observed during heating in an inert atmosphere.