The penta-ammine vanadium (III) borohydride, i.e. V(BH4)(3)center dot 5NH(3), was successfully synthesized via ball-milling of VCl3 center dot 5NH(3) and LiBH4 in a molar ratio of 1:3. This compound was shown to release 11.5 wt% hydrogen with a H-2-purity of 85 mol% by 350 degrees C. To improve the dehydrogenation purity of V(BH4)(3)center dot 5NH(3), Mg(BH4)(2) with various molar ratios was mixed with V(BH4)(3)center dot 5NH(3) to synthesize expected ammine metal-mixed borohydrides, among which the formed VMg(BH4)(5)center dot 5NH(3) was indexed to be a monoclinic unit cell with lattice parameters of a = 19.611 angstrom, b = 14.468 angstrom, c = 6.261 angstrom, beta = 93.678 degrees and V = 1772.75 angstrom(3). Dehydrogenation results revealed that the Mg(BH4)(2) modified V(BH4)(3)center dot 5NH(3) system presents significantly enhanced dehydrogenation purity. For example, in the case of V(BH4)(3)center dot 5NH(3)/2Mg(BH4)(2) sample, 12.4 wt% pure hydrogen can be released upon heating to 300 degrees C. Further investigation on the dehydrogenation mechanism of the VMg(BH4)(5)center dot 5NH(3) system by isotope tagging revealed that the interactions of homo-polar BH units also participated throughout the dehydrogenation process (onset at 75 degrees C) as complementary to the prime combination of BH center dot center dot center dot HN. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.