In the title compound, C(20)H(22)O(8)S, the dihedral angle between the two benzene rings is 81.6 (3)°. The benzene-connected portions of the alk-oxy substituents are almost coplanar with their respective rings [C-C-O-C torsion angles of 174.77 (17) and -178.5 (4)°]. One of the butanoic acid groups is disordered over two conformations with a site-occupancy ratio 0.719 (6):0.281 (6). In the crystal, pairs of O-H⋯O hydrogen bonds link the mol-ecules into infinite zigzag chains along [130].