The covalent modification of the metal-organic framework IRMOF-3 has been achieved using acetic anhydride. Mass spectrometry and 1H NMR evidence shows that the free amine groups in IRMOF-3 are converted to amides in high yields. Control experiments and X-ray diffraction studies suggest that this reaction occurs on the intact framework with preservation of crystallinity. This finding opens up new possibilities for the preparation of novel MOFs using a postsynthetic modification strategy.