In the phase diagram of mixtures of magnesium nitrate and acetate hydrates, a maximal melting temperature of ca. 83 o C has been found at a composition near 1:3. Crystals have been grown at this composition from both the melt and a saturated aqueous solution. Single-crystal diffraction shows that a double salt of the stoichiometry [Mg 2 (CH 3 CO 2 ) 3 (H 2 O) 6 ] + NO 3 - is formed. This formula is confirmed by elemental analysis. The crystallographic data refined to R w =0.049 are: space group P6 3 , with lattice parameters a=8.977(1)Å and c=12.553(2)Å, there being two formula units per unit cell. The Raman spectrum of this salt, showing the nitrate group to be isolated, is in agreement with the X-ray data. The structure of this compound is quite different from those of the parent compounds.