Magnesium vanadate, Mg2V2Ox, and promoted systems of the composition Mg4V2M2Ox, where M = Mo, W, B, Al, Ga, Sb and (SbP), were synthesized, characterized and evaluated for the oxydehydrogenation of n-butane in the temperature range 300-500 degrees C and atmospheric pressure. The selectivities to butene/butadiene of the majority of the promoted catalysts are, at 500 degrees C and constant n-butane conversion (2%), substantially higher than those of the base system : Sb(87), SbP(82), B(82), Mo(68), Al(62), base(55), W(50). The superior selectivities of the antimony and boron containing compositions are probably primarily due to the formation of ternary Mg-V-Sb-oxide (Mg1-xSbV2/3xO3.5) and Mg-V-B-oxide (Mg1-xBV2/3O2.5) phases, contributing to the structural site isolation of the paraffin activating vanadium sites; the stabilization of the V4+ or lower oxidation states; and their possible interaction with an inferred supra-surface Sb-oxide phase. Highest butene/butadiene per pass yields were obtained with a Mg4V2Ga2Ox catalyst (5.2% at 7.9% conversion, at 500 degrees C, 1 atm, C-4(0)/O-2/N-2=5/1/20, and WHSV=13 h(-1)). The only composition yielding measurable amounts of furan was the W promoted catalyst.