A new method for the synthesis of hydrated aluminum phosphate, AlPo(4)(.)1.5H(2)O, in a semibatch reactor is reported. The synthesis involves direct reactive crystallization induced by pH adjustment to a predetermined supersaturation level. Precipitation was conducted in sulfate-based solutions with AI(III):P(V) molar ratio of 1 and at 95 degrees C. Homogeneous precipitation tests led to the definition of the metastable zone of aluminum phosphate. This zone was used to design the supersaturation-controlled seeded test. XRD, TGA, and chemical analysis confirmed the product to be AlPO(4)(.)1.5H(2)O, SEM and particle size analysis revealed the product to consist of aggregates of submicron-size crystallites of 3.5 mu m average size (homogeneously formed product) grown to 16.5 mu m after three cycles of seeded crystallization.