The structure of the title compound, known as quinine sulfate dihydrate, 2C(20)H(25)N(2)O(2) (+)·SO(4) (2-)·2H(2)O, was previously reported by Mendel [Proc. K. Ned. Akad. Wet. (1955), 58, 132-134], but only the [010] projection was determined. Hence, we have redetermined its crystal structure at 100 K using three-dimensional data. The asymmetric unit consists of a quininium cation, viz. (R)-(6-meth-oxy-quinolinium-4-yl)[(1S,2S,4S,5R)-5-vinyl-quinuclid-in-ium-2-yl]methanol, one half of a sulfate anion and a water mol-ecule. The S atom occupies a special position on a twofold axis. The packing is characterized by infinite columns, consisting of sulfate anions and water mol-ecules, linked through hydrogen bonds along the b axis, and further stabilized by hydrogen bonds to quininium cations. The quininium cations inter-act further through C-H⋯O and C-H⋯π inter-actions.