The selective leaching of zinc over iron from basic oxygen steelmaking (BOS) filter cake was investigated using mixtures of butyric acid with acetic and/or propionic acid, HCl, or H 2 SO 4 . The main focus was to assess the effects of other acids on the zinc and iron leaching in butyric acid. The results show that slight reduction of zinc leaching and increase in iron leaching may take place with addition of acetic and propionic acids, causing a minor decrease in the selectivity of zinc leaching over iron. Intermittent addition of H 2 SO 4 and HCl was required to control the pH to be not less than that of pure butyric acid solution. Overall, an excellent selectivity can be maintained with the butyric acid–other acid mixtures. The results demonstrate the feasibility to use raw butyric acid without purification or waste butyric acid containing other impurity acids for leaching of zinc from the BOS filter cake to be recycled. Using high content of H 2 SO 4 in leaching is not recommended because it has a more detrimental effect on the selectivity of zinc leaching and may cause deposition of CaSO 4 in the leaching residue which may increase the SO 2 or CO 2 emission in later high-temperature recycling processes.