The interactions of various acids, such as hydrochloric, hydrobromic, nitric, perchloric, and tetrafluoroboric acids with 2-(1H-imidazole-2-ylthio)-3-methylnaphthalene-1,4-dione (L) enhance the intensity of the fluorescence emission of L. Exceptionally, the interaction of hydrogen bromide with L not only enhances the emission intensity, but also leads to a sharp characteristic emission at λ=480 nm (λex=350 nm), which is different from the other acids. Bromide-ion recognition by protonated L is explained on the basis of a tautomeric equilibrium. The Stokes shifts were calculated for each case and they were dependent on the anions and, in general, were found at λ>100 nm. Fluorescence lifetimes were measured and it was shown that two independent paths operated for the emission processes in solutions of the salts of L. Various salts of L with the general composition [HL][X] (in which X=Cl− (1), Br− (2), NO3− (3), ClO4− (4), and BF4− (5)) are structurally characterized. The coordination environment of the corresponding anion in these salts in the solid state is guided by electrostatic N+H⋅⋅⋅O interactions.