The structure of the pseudo-merohedrally twinned crystal of the 1:1 proton-transfer compound of 5-sulfosalicylic acid (3-carboxy-4-hydroxybenzenesulfonic acid) with 4-aminopyridine: 4-aminopyridinium 3-carboxy-4-hydroxybenzenesulfonate sesquihydrate has been determined at 180 K and the hydrogen-bonding pattern is described. Crystals of the compound are monoclinic with space group P2₁/c, with unit cell dimensions a = 35.2589(8), b = 7.1948(1), c = 24.5851(5) 脜, β = 110.373(2)掳, and Z = 16. The monoclinic asymmetric unit comprises four cation–anion pairs and six water molecules of solvation with only the pyridinium cations having pseudo-symmetry as a result of inter-cation aromatic ring π–π stacking effects. Extensive hydrogen bonding gives a three-dimensional framework structure.