Ammonia decomposition for hydrogen production was studied using cobalt catalysts supported on different Mg mixed oxide systems (MgAl, MgCe and MgLa) to elucidate the influence of support composition on the activity of these catalysts. For this purpose three supports of Mg to X ratio (X = Al, Ce or La) equal to two were prepared. These supports were applied to synthesis 5 wt% cobalt samples by impregnation. The catalytic performance was evaluated in the temperature range of 300-550 degrees C at atmospheric pressure. It was found that 5CMLa-2 (5 wt% Co impregnated on MgLa support with Mg:La = 2:1) has the highest activity among the other catalysts and the decreasing order of NH3 conversion as follows: 5CMLa-2 > 5CMCe-2 5CMA1-2. Another series of mixed Mg-La oxide supports with different Mg/La molar ratios (Mg/La = 1, 2,3,5,9 and 14) were prepared and impregnated with 5 wt% cobalt. The prepared catalysts were characterized by BET, XRD, TPR, XPS and CO chemisorptions techniques. Investigation of the effect of La content in the mixed oxide support showed that the 5 wt% cobalt with Mg/La ratio 5 catalyst was the most active. This could be attributed to enhance the interaction between Mg and La which leads to suitable basicity for ammonia decomposition reaction. The increase of MgO content creates high surface area, high active metal area and high surface lattice oxygen (i.e. O2-). From TPD data the basic sites become stronger in the Mg-La mixed oxide at Mg/La = 5 ratio. 5CMLa-5 (5 wt% Co on MgLa with ratio 5:1) is showing highest activity among all catalysts with other Mg/La ratios. The enrichment in activity of 5CMLa-5 catalyst could be attributed to increase in surface area, metal dispersion, easily reducible Co species and high basicity. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.