1-Ethyl-3-methylimidazolium acetate (EMIMAc) is an ionic liquid (IL) often investigated as a solvent, especially in the context of biopolymers and biomass pretreatment. A reduced solvent efficacy occurs upon the addition even of low amounts of water to EMIMAc. Molecular mechanisms have not yet been fully understood. It is expected that the functionality as hydrogen bond donor and acceptor is key for the solvent–solute interactions. In this work, we analyze the solvent efficacy of EMIMAc in terms of hydrogen–deuterium (H/D) exchange at the C(2)-position in mixtures with water or acetic acid added as proton donors. Low-field NMR spectroscopy and deuterated solvents are used for a time-resolved evaluation of H/D exchange reactions. The H/D exchange is also modeled to explore changes in the reaction kinetics as a function of the mixture composition. The significant difference in calculated rate constant values among the concentration regimes shows that the chosen model equations of a possible pseudo-first-order and second-order reaction mechanism including water dissociation do not cover all interaction phenomena that influence the exchange in the individual concentration ranges. However, the modeling also indicates that the investigated interaction of EMIM⁺ and Ac⁻ remains constant for concentrated IL mixtures containing 70 mol% of EMIMAc in water up to diluted mixtures as low as 30 mol% EMIMAc. This exemplifies the change between ions strongly associated in networks in concentrated mixtures suitable for biomass pretreatment and the much less associated anion–cation pairs in diluted mixtures which leads to the decreased efficiency of EMIMAc with increasing water content.