A postsynthetic treatment with diluted solutions of the inorganic HCl or H₂SO₄ acids was applied to functionalize the eight connected Zr-based metal-organic framework DUT-67 (DUT = Dresden University of Technology). During the treatment, it is possible to remove and exchange the pristine modulator (formate) by Cl^- or SO₄ ^2- anions. The position of the chlorine in the crystal structure of DUT-67 after HCl treatment could be determined by single-crystal X-ray diffraction analysis. Moreover, by means of the acidic treatment the polarity of the network as well as its Brønsted acid strength are increased, which have a crucial impact on the catalytic performance. The improved catalytic activity of the acid-treated materials was demonstrated in the esterification of levulinic acid with ethanol.