The hydroplasticization of coatings of acrylic copolymers with different amounts of methacrylic acid (MAA) was investigated to clarify the role of carboxylic acid functionalities on the change in polymer mobility due to water uptake. The coating T g as a function of water uptake was studied using dynamic mechanical analysis. The T g 's decreased with increasing water content, confirming the plasticizing effect of water on the coatings. At relative humidities RH between 0 and 60% the coating T g shows a sharper decrease than at higher RH, an effect that increases with increasing MAA content. This behavior is attributed to the presence of dimers of carboxylic acid in the coatings, which is also observed with FTIR-ATR analyses. Due to water uptake, the dimers are disrupted and form “open” dimers where carboxylic acid groups remain in close proximity and are connected through water molecules. With 1 H NMR relaxometry, two T 2 relaxation times are found, representing two hydrogen pools with different mobilities. Both mobilities increase with increasing water content, indicating the presence of polymer domains with different hardness. Correlating the T 2 relaxation times with the coating T g 's shows that at higher MAA content the proton mobility as a function of T g of the soft domains increases with increasing MAA content. Since the polymer proton mobility, and hence the polymer mobility, is expected to scale with the polymer T g , it is hypothesized that harder domains are present in the coatings, which are not visible in the Ostroff-Waugh decays due to the fast relaxation behavior of these protons.