High-purity polysilicon granules are prepared by depositing reaction gas on silicon granules in a fluidized bed reactor having: a reactor space comprising at least two zones lying one above the other, the lower zone weakly fluidized by introduction of a silicon-free gas into silicon granules in the lower zone by a plurality of individual dilution gas nozzles, and a second, reaction zone directly abutting the lower zone, the reaction zone heated via its outwardly bounding wall, introducing silicon-containing reaction gas as a vertical high speed gas jet into the reaction zone by reaction gas nozzle(s), forming local reaction gas jets surrounded by bubble-forming fluidized bed, gas decomposing leading to particle growth, wherein the reaction gas has fully or almost fully reacted to chemical equilibrium conversion before reaching the wall or bed surface.