In a reactor for the decomposition of a silicon-containing gas, provision is made, to avoid silicon deposition on an inner wall of a reactor vessel, for at least one catalytically active mesh to be provided within a reaction chamber between at least one gas feed line and the inner wall (4). The mesh accelerates the thermal decomposition of the gas and reduces the deposition of silicon on the inner wall. Also described is a process for the preparation of silicon using the reactor according to the invention and the use in photovoltaics of the silicon prepared.