Schiff base 4-[(2-hydroxy-3-methoxybenzylideneamino)-N-(5-methylisoxazol-3-yl)benzene-sulfonamidehas 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, H-1 NMR, C-13 NMR, HET-COR 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 P2(1)/c with a = 8.2694(7), b = 8.3453( 5), c = 26.260( 2) angstrom, beta = 97.142(7)degrees, V = 1798.1(2) angstrom(3) , D-x = 1.431 g cm(-3), R-1 = 0.0529 and wR(2) = 0.1370 [ I > 2 sigma(I)], respectively.