Magnesia - carbon nanotubes (abbreviated as MgO - CNTs) nanocomposites were prepared by impregnation of CNTs with Mg(NO3)(2) .6H(2)O in ethanol solution, followed by drying at 353 K and calcination at 873 K, respectively. The nanocomposites are thermally more stable than CNTs in a H-2 flow. The use of the nanocomposites as support yielded more efficient Ru catalysts for the generation of COx-free hydrogen from NH3 decomposition.