The dissolution kinetics of colemanite in phosphoric acid solutions was studied. The effects of particle size, temperature, acid concentration, solid-to-liquid ratio, and stirring speed on the dissolution rate were determined. It was observed that the dissolution rate increased with decreasing particle size and solid-to-liquid ratio and with increasing temperature, but stirring speed had no effect on it. The dissolution rate increased up to an acid concentration of 19.52% (by wt) and then decreased with increasing acid concentration. The dissolution kinetics of colemanite were examined according to the heterogeneous and homogeneous reaction models for the acid concentration range of 1.43-19.52% (by wt) of acid concentration, and it was found that the dissolution rate was controlled by surface chemical reaction. The activation energy of this process was determined to be 53.91 kJ mol(-1).