PURPOSE: To prevent the formation of a film at a combustion chamber wall in the preparation of highly dispersive silica by reacting silicon compounds with gases forming water on combustion in a flame and discharging gases into the combustion chamber by means of gas guns.

CONSTITUTION: Highly dispersive silica is prepared by reacting gaseous or vaporizable silicon compounds (e.g. SiCl4) in a mixture with gases forming water on combustion and air or oxygen in a flame in a combustion chamber 1. The gas fed via a gas inlet 3 (e.g. high-pressure air) is poured into the combustion chamber 1 by means of gas guns 2a, 2b, 2c to prevent the deposition of SiO2 particles at the combustion chamber 1 inner wall. By this procedure the formation of a SiO2 film at the combustion chamber 1 inner wall is prevented and the quality of the product i.e., the highly dispersive silica is improved. Furthermore the efficiency of the process is improved since an operation with high particle concn. is made feasible.