MgO supports were prepared by decomposing various precursors at various temperatures and for various durations. The effect of the precursor and conditions employed to prepare MgO on the conversions of CO2 and CH4, and the CO yield over the reduced NiO/MgO catalyst in the CO2 reforming of CH4 was investigated. The yield and the conversions increase with reaction time and attain constant values. The latter values as well as the induction lime depend upon the precursor and conditions employed to prepare MgO.