The dissolution kinetics of ulexite in ammonium sulfate solutions was investigated. The effects of ammonium sulfate concentration, particle size, solid/liquid ratio, stirring speed, and reaction temperature on the dissolution rate have been evaluated. It was found that the dissolution rate increased with increasing ammonium sulfate concentration, stirring speed, and reaction temperature. However, increasing the particle size and solid/liquid ratio decreased the dissolution rate. Experimental data were examined by the heterogeneous and homogeneous models. The heterogeneous diffusion-controlled ash or product layer may describe the dissolution rate. The following mathematical model was used to represent the reaction kinetics: 1 - 3(1 - X)(2/3) + 2(1 - X) = 3.9 x 10(7) (CD-0.85)-D-0.64(S/L)(-1.06)omega(0.40)e(-10050/T)t, where X is the fractional conversion, C the ammonium sulfate concentration, D the particle size, S/L the solid-to-liquid ratio, omega the stirring speed, T the reaction temperature, and t the reaction time.