Abstract: It is a very slow transformation from paratungstate Ainto Paratungstate B, nevertheless, it can be completed within several hours under the catalysis of silica. When the reaction finished, the pHvalue of the solution rose up to 8.1 from its initial value 6.7. Under the final pHcondition, no polymerization of molybdenum occured. The difference between the abilities of forming polyanions of between tungsten and molybdenum can be used for their separation. Adding guanidine to the solution containing tungsten and molybdenum, tungsten precipitated in the form of guanidine paratungstate B, while molybdenum remained in the solution as normal molybdate.

Key words: Paratungstate A, Paratungstate B, Guanidine, Separation of W and Mo