Gas loading and release from MFU-4 and MFU-4-Br were investigated using xenon and krypton as model gas guests, with kinetic diameters of 3.96 and 3.60 Å, respectively. Our aim was to correlate the influence of the ratio between the guest kinetic diameter and the pore aperture size on the gas loading. Depending on the guest-material pair, the guest was between 1.42 to 1.84 times larger than the MOF pore aperture and for the ratio, the following trend could be concluded: Kr/MFU-4 < Xe/MFU-4 < Kr/MFU-4-Br < Xe/MFU-4-Br (Table ).