Murdochite-type (Mg6-xAlx)MnO8 (0 less than or equal to x less than or equal to 0.6) was synthesized at low temperature (700 degrees C) with the sol-gel process. The specific surface area, crystallite size, catalytic activity for the CO oxidation, and adsorption isotherms of oxygen on (Mg6-xAlx)MnO8 were measured. The increase of the specific surface area with increasing Al3+ ion content was explained by the decrease of the particle size. The catalytic activity increased with increasing Al3+ ion content, and this increase was caused by the valence deviation from the Mn4+ ion to the Mn-3.5+ ion.