The very large expansion upon adsorption of liquids, up to 160% in cell volume, has been observed in the iron(iii) trans,trans muconate MIL-89 [(MIL = Material Institut Lavoisier]. The structure of lutidine-containing MIL-89 (lutidine = 2,6-dimethylpyridine) has been determined using X-ray powder diffraction and computer simulations. Finally, the consequences of adsorption of various polar and apolar liquids have been evaluated using ex situ synchrotron X-ray powder diffraction data and reveals that the swelling behavior of MIL-89 is selective but slightly different from the MIL-88 analogues.