Single-crystal X-ray studies of calcium bis-(2,4-dinitrophenoxide) heptahydrate (1)[a= 10.627(5), b= 25.536(11), c= 7.922(2)Å, β= 92.69(3)°, D_calc.= 1.646 g cm^–3, Z= 4, monoclinic, space group P2₁/a, R= 0.067, R_w= 0.097] and magnesium bis-(2,4-dinitrophenoxide) octahydrate (2)[a= 13.473(4), b= 12.969(4), c= 6.670(2)Å, β= 104.98(2)°, D_calc.= 1.577 g cm^–3, Z= 2, monoclinic, space group P2₁/c, R= 0.086 R_w= 0.123] allow a comparison of binding patterns. In structure (1) calcium is bonded to eight oxygen atoms: two from one of the 2,4-dinitrophenoxide anions (the phenoxy oxygen and one nitro oxygen atom) and six from water molecules, one of which bridges two adjacent calcium atoms. The remaining phenoxide is hydrogen-bonded to the co-ordinated water molecules. In structure (2) magnesium is octahedrally co-ordinated to six water oxygen atoms. The 2,4-dinitrophenoxide anions are not directly bound to magnesium, but are hydrogen-bonded to the water molecules.