Abstract Water-soluble derivative of formononetin,[Na (H 2 O) 1.5][Na (H 2 O) 3.5] X 2· 2H 2 O (X= 4′-methoxy-7-hydroxyisoflavone-3′-sulfonate) was synthesized by sulfonation reaction. It was characterized by IR, 1 H NMR and single-crystal X-ray diffraction analysis.