Schiff base 4-[(2-hydroxy-3-methoxybenzylideneamino)-N-(5-methylisoxazol-3-yl)benzene-sulfonamide has been synthesized from the reaction of 4-amino-N-(5-methylisoxazol-3-yl)benzenesulfonamide(sulfamethoxazole) with 2-hydroxy-3-methoxybenzaldehyde. It has been characterized by elemental analysis, MS, IR, ¹H NMR, ¹³C NMR, HETCOR and UV–Visible techniques. The structure of it also has been examined crystallographically. For the compound exist as dominant form of enol-imines in both the solid state and the solutions. It crystallizes in the monoclinic space group P2₁/c with a = 8.2694(7), b = 8.3453(5), c = 26.260(2) Å, β = 97.142(7) °, V = 1798.1(2) Å³, D _x  = 1.431 g cm⁻³, R ₁ = 0.0529 and wR ₂ = 0.1370 [I > 2σ(I)], respectively.

The tautomerism in the Schiff base ligands plays an important role for distinguishing their photochromic and thermochromic characteristics. Both phenomena is associated with a proton transfer (enol-imine, O–H···N, keto-amine, O···H–N).