Abstract: Alkali metal oxide-metal oxide mixed oxide powder in the form of aggregates of pore-free primary particles, comprising from 0.005 to 5% by weight of at least one alkali metal oxide, which has a BET surface area of from 100 to 350 m/g, has a specific DBP number, expressed as DBP number per square metre of specific surface area, greater than or equal to that of a powder which has only the metal oxide component, has the alkali metal oxide distributed in the core and on the surface of the primary particles. Silicone rubber comprising the alkali metal oxide-metal oxide mixed oxide powder.