The hydration of carboxylic acids in dilute aqueous solutions is important for our understanding of their functioning in the biochemical-contextjiere we apply vibrational spectra of HDO isotopically diluted in H2O to study this phenomenon, using the difference spectra method for analysis and interpretation of the results. The spectra of EDO affected by formic, acetic, and propionic acid display characteristic component bands, significantly red-shifted from the bulk HDO band position. The appearance of these component bands is linked with isotopic substitution on the carboxylic acid molecule, which forms a short and strong hydrogen bond with a water molecule. Additionally, a charge separation due to the proton transfer in the neutral form of the complex leading to a contact ion pair formation may be inferred from the affected HDO spectra. Apart from the contraction of the principal acid-water hydrogen bond, it results in other major structural changes in the hydration shell, as revealed by density functional theory (DFT) calculations of optimal geometries of aqueous clusters of the studied acids.