In this study, the importance of surface charge in the precipitation of probertite (NaCaB5P9 . 5H(2)O) in borax solution was shown. By controlling the surface charge with polyelectrolytes, the removal of calcium ions from the borax solution in the form of probertite, which is the most difficult impurity to remove, was studied. The use of only 5 ppm cationic polyelectrolyte and 0.2% of probertite seed crystals was sufficient to reduce the calcium ion concentration to the 200 ppm level in 2 hours. By comparison with the 50-60 hours precipitation time from clear solution to the same level of calcium concentration, it was concluded that polyelectrolytes can be used as tailor-made additives to control precipitation.