Both 7-carboxylato-8-hydroxy-2-methylquinolinium monohydrate, C11H9NO3.H2O, (I), and 7-carboxy-8-hydroxy-2-methylquinolinium chloride monohydrate, C11H10NO3+.Cl-.H2O, (II), crystallize in the centrosymmetric P-1 space group. Both compounds display an intramolecular O-H...O hydrogen bond involving the hydroxy group; this hydrogen bond is stronger in (I) due to its zwitterionic character [O...O = 2.4449 (11) A in (I) and 2.5881 (12) A in (II)]. In both crystal structures, the HN+ group participates in the stabilization of the structure via intermolecular hydrogen bonds with water molecules [N...O = 2.7450 (12) A in (I) and 2.8025 (14) A in (II)]. In compound (II), a hydrogen-bond network connects the Cl- anion to the carboxylic acid group [Cl...O = 2.9641 (11) A] and to two water molecules [Cl...O = 3.1485 (10) and 3.2744 (10) A].