The present invention relates to a process for producing spherical submicron particles of amorphous silicon dioxide, in which silicon dioxide and a reducing agent is injected into a reaction vessel of zirconium oxide in molten state, said zirconium oxide is serving as a heat reservoir, in which silicon dioxide reacts with the reducing agent producing a silicon-sub-oxide vapor, said silicon-sub-oxide vapor is oxidized into said spherical submicron silicon oxide particles.