PURPOSE: To provide a colloidal soln. of a W cpd. stable at high concn., effective as a sizing agent for fiber, paper, etc., and contg. the W cpd., a Si cpd. and alkali metal ions or NH4 ions in a predetermined molar ratio.

CONSTITUTION: An aq. soln. of M2WO4 (M; an alkali metal), e.g. Na2WO4 or (NH4)2WO4 is treated by a cation exchange method to remove the alkali metal or NH3. The soln. thus treated in mixed with an alkali silicate such as No.3 water glass, ammonium silicate or a these cpds. forming substance in the following molar ratio to prepare a desired colloidal soln. When each component is turned into WO3, SiO2 and M2O, the molar ratio of them is 4W15:2W5:1, esp. 6W12:2W4:1. The resulting colloidal soln. is effective in many fields as a sizing agent for fiber and paper, raw material for novel catalyst, material for incombustible coating, etc