A series of alkali metal and ammonium salts of 6-aminonaphthalene-2-sulfonic acid have been prepared by reaction of the metal hydroxide (or ammonia) and the substituted naphthalenesulfonic acid in water or aqueous ethanol. The structures were determined by single-crystal X-ray diffraction techniques. Crystal data: K(H2NO10H6SO3)•H2O, orthorhombic, Pca21 a = 27.139(3) Å,b = 6.172(3) Å, c = 7.135(3) Å, V= 1195(1) Å3, Z = 4, R(F0) = 0.039 for 659 observations (I > 3a(I)), 154 variables; Na(H2NC10H6SO3)•2H2O, orthorhombic, Pna21, a = 11.200(4) Å,b = 7.916(4) Å, c = 26.752(3) Å, V= 2372(2) Å3, Z = 8, R(F°) = 0.045 for 1813 observations (I > 3a(I)), 325 variables; NH4(H2NC10H6SO3)•H2O, orthorhombic, Pbca, a = 12.231(5) Å, b = 26.994(4) Å, c = 7.208(5) Å, V = 2380(2) Å3, Z = 8, R(F°) = 0.056 for 1145 observations (I > 3σ(I)), 190 variables. All of the structures contain layers of sulfonate anions in which the long axes of the naphthalene groups are approximately parallel to the stacking direction of the layers. The cations lie between the organic layers bonded to the sulfonate oxygen atoms and one or more water molecules. The structures differ in the detailed packing patterns due to the different sizes and coordination behaviors of the cations. The structure of the monosodium salt of 3-ammonionaphthalene-1,5-disulfonic acid has also been determined: Na(H3NC10H5(SO3)2)•H2O, triclinic, P1, a = 10.823(3) Å, b = 12.885(5) Å, c = 5.127(1) Å, α = 100.33(3)°, β = 101.65(2)°, γ = 66.19(2)°, V = 636.9(4) Å3, Z = 2, R(F°) = 0.042 for 1311 observations (I > 3cr(I)), 223 variables. The sulfonate anions stack face-to-face in infinite columns which are linked into layers by the sodium ions. These layers are held together by hydrogen bonds between the ammonium and sulfonate groups and between water molecules coordinated to the sodium ions. The structures of these compounds are compared with those of previously studied metal sulfonates.