Abstract The forms of zinc in the blast furnace and steelmaking sludge were determined applying the sequential extraction method of chemical speciation. In the blast furnace sludge zinc is largely present in the ion-exchange and superficially occluded form and that is why its separation by acid leaching may be successful. These assumptions were proved when studying the heterogeneous acid-base equilibrium in discontinual static leaching under mild acidic conditions - c(HCl) max. 0.1 mol.dm-3, when 77 % of zinc were extracted and iron practically did not pass into solution. As for the steelmaking sludge, an unambiguous correlation between the chemical speciation and acid leaching has not been found. When leaching this material, the buffering equilibrium is established at pH ? 5.5 and both zinc and iron dissolve simultaneously with the growing acidification. This buffering equilibrium establishing is connected with the presence of iron in the lower oxidation states, especially in the Fe(II) form.