Colemanite is one of the most important boron minerals used for Production of boric acid. Boric acid is produced by the reaction of colemanite (2CaO.3B(2)O(3).5H(2)O) with sulfuric acid in a heterogeneous solid-liquid reaction leading to crystallization of gypsum as a by product. Gypsum is removed by filtration and boric acid, which has a high solubility in water, is crystallized by cooling the filtrate. The effects of ultrasound on the dissolution of colemanite in H2SO4 solution and on the precipitation of the gypsum during the reaction were studied in a batch reactor at 85 degrees C in the absence and presence of ultrasound The stirring rates were chosen as 600 rpm (for 250-355 and 500-600 mu m) and 800 rpm (1000-1180 mu m) to provide a homogeneous suspension during the reaction. The boric acid and calcium ion concentrations in the solution were determined as a,function of time. The results showed that ultrasound enhances the dissolution of colemanite and affects the size of gypsum crystals. The calcium ion concentration in boric acid solution is also affected in the presence of ultrasound The colemanite having smaller particle size dissolved faster than the bigger particles.