The hydration of para-aminobenzoic acid (PABA) and its anion (PABA?) in aqueous solution was investigated by 1D- and 3D-RISM integral equation methods. It was found that the first hydration shell of PABA and PABA? consists of ~ 28.4 and 27.2 water molecules, respectively. The average number of water molecules close the amino group is ~ 6.6 for PABA and ~ 7 for PABA?. The total number of H-bonds formed by the amino group is ~ 1.8 for PABA and ~ 1.9 for PABA?. The carboxyl (COOH) group forms on average ~ 2.9 hydrogen bonds, whereas carboxylate (COO?) has ~ 5.5. Thus, deprotonation of PABA is facilitated by increased hydration of the carboxylate moiety. For PABA and PABA? ~ 1.2 ¦Ð-coordinated water molecules are preferably found above and below the plane of the aromatic ring at a distance of ~ 0.3 nm from the ring center with slightly stronger binding for PABA?.