5-Sulfosalicylic acid (5-SSA) and 3-amino-pyridine (3-APy) crystallize in the same solvent system, resulting in two kinds of 1:1 proton-transfer organic adduct, namely 3-aminopyridin-ium 3-carb-oxy-4-hydroxy-benzene-sulfonate monohydrate, C5H7N2 +·C7H5O6S-·H2O or 3-APy·5-SSA·H2O, (I), and the anhydrous adduct, C5H7N2 +·C7H5O6S- or 3-APy·5-SSA, (II). Both compounds have extensively hydrogen-bonded three-dimensional layered polymer structures, with inter-layer homo- and heterogeneous π-πinter-actions in (I) and (II), respectively. ©International Uni...