The effect of the substitution of manganese by magnesium on the activity and physicochemical properties of a LaMn1-xMgxAl11O19 hexaaluminate catalyst was investigated. The samples were prepared using a coprecipitation method and characterized by X-ray diffractometry (XRD), hydrogen temperature-programmed reduction (TPR), oxygen temperature-programmed desorption (TPD), and nitrogen adsorption. The results show that the Mg-substitution enhances the catalytic activity. A sample with a composition of LaMn0.7Mg0.3Al11O19 has a very good activity and T-10% = 704 K. XRD analysis suggests that the partial substitution of manganese by magnesium effectively suppresses the crystal growth along the [001] direction, which leads to an improved surface area. Although the increase in activity is partially attributable to the high surface area, TPD and TPR results confirm that the substitution also affects the oxygen adsorption property and the oxidation state of Mn ions in the hexaaluminate lattice. The reducibility, the amount of Mn3+ species, and the lattice oxygen mobility also are improved.