The low-temperature (130 K) structure of the 1:1 proton-transfer compound of brucine with toluene-4-sulfonic acid (systematic name: 2,3-dimethoxy-10-oxostrychnidinium toluene-4-sulfonate trihydrate), C23H27N2O4+烷H7O3S-糈2O, has been determined. The asymmetric unit contains two cations, two anions and six molecules of water. Brucinium cations form the familiar undulating head-to-tail ribbon structures, which associate with the toluene-4-sulfonate anions and the water molecules in the interstitial cavities through hydrogen-bonding associations involving all available donor and acceptor atoms on all species. The result is a framework polymer structure.