A combined electron diffraction and hydrogen adsorption study on a nickel–magnesia catalyst with 36% nickel is reported. A comparison of the adsorption results with those on the previously reported 79% and 18% catalysts indicates the possibility of the unreduced nickel in the catalysts taking part in high-temperature fast adsorption. The electron optical studies show that there are structural changes taking place during the reduction treatment the catalyst undergoes. However, diffraction study in situ appears to be necessary to show, beyond doubt, the presence of reduced and unreduced nickel on the catalyst's surface.