In the phase diagram of mixtures of magnesium nitrate and acetate hydrates, a maximal melting temperature of ca. 83degreesC 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(CH3CO2)(3) (H2O)(6)](+) NO3- 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) Angstrom and c = 12.553(2) Angstrom, 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. (C) 2003 Elsevier Science B.V. All rights reserved.