The kinetics of hydrolytic precipitation of potassium vanadates KxV12O31-delta. nH(2)O (0 < x less than or equal to 2.25; 0 < delta less than or equal to 1.15) from a KVO3 solution at pH = 1.7 and 80 degreesC in the presence of K2SO4 and VOSO4 is controlled by second and third order reactions respectively occurring on the precipitate surface. This difference is due to the effect of VO2+ ions on the polymer anion [V12O31-delta](n) formation. Potassium vanadates KxV12O31-delta. nH(2)O and KxV3O8-delta.H2O (1.02 less than or equal to x less than or equal to 1.91; 0 less than or equal to delta less than or equal to 0.22) can be used as vanadium-containing precursors for sulfuric acid catalysts.