1-(3-Carboxyl-atophen-yl)-4,4'-bipyridin-1-ium dihydrate.

In the crystal structure of the title compound, C17H12N2O2·2H2O, the carboxyl-ate group is linked via O-H⋯O hydrogen bonds to two water mol-ecules. The crystal packing is best described as parallel layers (viewed along the a axis) of viologen and water mol-ecules associated via O-H⋯O hydrogen bonds and π-π inter-actions, with a centroid-centroid separation of 3.8276 (9) Å.