Silicon-aluminum mixed oxide powder having a weight ratio of (Al2O3/SiO2)ttl in the total primary particle of from 0.003 to 0.05, a weight ratio (Al2O3/SiO2)surface of the primary particles in a surface layer having a thickness of about 5 nm which is less than in the total primary particle and a BET surface area of from 50 to 250 m2/g. It is prepared by igniting one or more silicon compounds selected from the group consisting of CH3SiCl3, (CH3)2SiCl2, (CH3)3SiCl and (n-C3H7)SiCl3, a hydrolysable and oxidizable aluminum compound, at least one fuel gas and air and burning the flame into a reaction chamber, subsequently separating the solid from gaseous materials and subsequently treating the solid with water vapor. The silicon-aluminum mixed oxide powder can be used as catalyst.