The title compound, C(20)H(23)N(4)O(6)PS, was synthesized by the reaction of N-(4-methoxy-benzyl-idene)-5-(4-nitro-phen-yl)-1,3,4-thia-diazol-2-amine and diethyl phosphite. The thia-diazole and nitro-substituted phenyl rings in the mol-ecule are approximately coplanar, the dihedral angle being 5.3 (2)°. The dihedral angle formed by the mean plane through all non-H atoms of both the thia-diazole and the nitro-substituted phenyl ring with the plane of the meth-oxy-substituted phenyl ring is 48.9 (2)°. In the crystal structure, mol-ecules form centrosymmetric dimers as a result of N-H⋯O bonds involving amine H and phosphine oxide O atoms.