(E)-2-[4-(Dimethylamino)styryl]-1-methylquinolinium 4-methoxybenzenesulfonate monohydrateThis paper is dedicated to the late Her Royal Highness Princess Galyani Vadhana Krom Luang Naradhiwas Rajanagarindra for her patronage of science in Thailand.

In the title compound, C20H21N2+·C7H7O4S−·H2O, the cation is nearly planar and exists in the E configuration. The cations and anions form individual chains along the b axis and are interconnected by weak C—H...O interactions. The 4-methoxybenzensulfonate anions are linked to water molecules through O—H...O hydrogen bonds, forming a three-dimensional network. The crystal structure is further stabilized by a C—H...π interaction involving the methoxyphenyl ring. The sulfonate anion is also involved in a weak intramolecular C—H...O interaction which generates an S(5) ring motif