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The molecular salt sulfamethoxazolium {or 4-[(5-methyl-1,2-oxazol-3-yl)sulfamoyl]anilinium methyl sulfate monohydrate}, C10H12N3O3S+ ·CH3O4S ·H2O, was prepared by the reaction of sulfamethoxazole and H2SO4 in methanol and crystallized from methanol–ether–water. Protonation takes place at the nitrogen atom of the primary amino group. In the crystal, N—H...O hydrogen bonds (water and methylsulfate anion) and intermolecular N—H...N interactions involving the sulfonamide and isoxazole nitrogen atoms, link the components into a tri-dimensional network, additional cohesion being provided by face-toface π-π interactions between the phenyl rings of adjacent molecules. A Hirshfeld surface analysis was used to verify the contributions of the different intermolecular interactions, showing that the three most important contributions for the crystal packing are from H...O (54.1%), H...H (29.2%) and H...N (5.0%) interactions [ABSTRACT FROM AUTHOR]