This work describes the studies on the volumetric method of water soluble boron determination in fertilizars, by titration of boric acid or borate with standardized NaOH solution, in presence of mannitol. The results allowed to conclude that it os necessary only 6.0 g of mannitol for the determination. This mannitol quantity is enough to titrate 2.5 miliequivalent or milimoles of H3BO3. Also, this present method is not influenced by urea and this application could by made in fertilizers containing up to 20% of urea.