The dissolution kinetics of calcined ulexite in ammonium chloride solutions at high solid-to-liquid ratios were investigated. In the experiments, calcination temperature, solution concentration, reaction temperature, and prehydration were chosen as parameters. It was observed that the dissolution rate increased with increasing calcination temperature, solution concentration, and reaction temperature, whereas it was not affected by pre-hydration. Employing graphical and statistic methods, the dissolution rate, based on homogeneous reaction model, can be given as: (1X(B2O3))(-1)-1= k(c(NH4Cl))(1.982)t. The activation energy for the dissolution of the ulexite sample calcined at 160 degrees C was found to be 84.04 kJ . mol(-1).