Leaching with sulfuric and hydrochloric acids for upgrading titanium slag obtained from the Qara-aghaj ilmenite concentrate to a high grade titanium dioxide (TiO₂) was studied. The titanium slag containing 72.7% TiO₂ and 7.8% Fe₂O₃ was leached at varying solid/liquid (S/L) ratio, particle size, leaching time and acid concentration. The optimum amount of S/L ratio, particle size, acid concentration and leaching time using both acids were determined as 1:4, -100 um, 8% and 2 h. Under optimum conditions, by using sulfuric acid, a titanium dioxide concentrate with 86.8% TiO₂ and 1.87% Fe₂O₃ was produced, while employing hydrochloric acid resulted in a concentrate containing 91% TiO₂ and 0.61% Fe₂O₃. The titanium dioxide concentrate prepared with hydrochloric acid, having acicular texture and by means of elongated separate particles; has less impurities than the one produced by sulfuric acid. The results demonstrate that hydrochloric acid dissolution of titanium slag and removing some impurities such as SiO₂ and MgO provide a product with good quality is prepared which is suitable as raw material for TiO₂ pigment production through the chloride process.