Polycrystalline silicon is prepared by thermally decomposing a reaction gas comprising hydrogen and a silicon-containing gas in a reaction chamber containing heated silicon, depositing additional silicon thereon, and forming an offgas; andseparating the offgas into a first fraction comprising trichlorosilane and lower boiling chlorosilanes, and a second offgas fraction comprising components having a higher boiling point than trichlorosilane;recycling the first offgas fraction to the reaction gas of a polycrystalline silicon deposition; andseparating the second offgas fraction into tetrachlorosilane and a high boiler fraction of high boilers, optionally also containing some tetrachlorosilane, and supplying the high boiler fraction to the reaction gas of a silicon deposition and heating the reaction gas to a temperature which ensures that the high boiler fraction is present in gaseous form on entry into the reaction chamber of the deposition reactor.