Metal–organic frameworks (MOFs) possess unique flexibility of structure and properties, which drives them toward applications as water adsorbents in many emerging technologies, such as adsorptive heat transformation, water harvesting from the air, dehumidification, and desalination. A deep understanding of the surface phenomena is a prerequisite for the target-oriented design of MOFs with the required adsorption properties. In this work, we comprehensively study the effect of functional groups on water adsorption on a series CAU-10-X substituted with both hydrophilic (X = NH₂) and hydrophobic (X = NO₂) groups in the linker. The adsorption equilibrium is measured at P = 7.6–42 mbar and T = 5–100 °C. The study of water adsorption by a set of mutually complementary physicochemical methods (TG, XRD in situ, FTIR, and ¹H NMR relaxometry) elucidates the nature of primary adsorption sites and water adsorption mechanisms.