The adsorption of CO₂ by a series of functionalized flexible MIL-53(Fe) solids has been evaluated through a combination of in situ X-ray power diffraction, adsorption calorimetry, IR spectroscopy and computer modelling. It appears that (i) strongly polar groups maintain the nonporous structure in its closed form due to strong intra-framework interactions and (ii) less polar functional groups allow only a modulation of the CO₂–framework interactions, in some cases with a disappearance of the initial intra-framework μ₂-OH⋯X hydrogen bonds, but do not interact directly with the CO₂ molecules.