In the title compound, C(24)H(25)O(7)P, the δ-valerolactonyl ring exists in a distorted screw-boat conformation with the diethoxy-phosphoryl substituent occupying an axial position. The latter adopts an almost syn-periplanar conformation around the P-C bond. The mol-ecules form centrosymmetric dimers connected by O-H⋯O hydrogen bonds.