Silicon-titanium mixed oxide powder produced by flame hydrolysis, which consists of aggregates of primary particles, with a BET surface area of 90.+-.15 m.sup.2/g, a titanium dioxide proportion of 50.+-.8 wt. % and an anatase/rutile ratio of 60:40 to 70:30.It is produced in that a mixture of silicon halide, titanium halide, hydrogen and primary air is ignited in a burner and the flame is burned into a reaction chamber closed off from the surrounding air, and secondary air and a gas, which increases the temperature in the reaction chamber by combustion and/or which slows down the cooling in the reaction chamber because of low heat transfer, are additionally introduced into the reaction chamber.It can be used in toner compositions.