Reaction chamer (10) for an epitaxial reactor comprising a belljar (14) made of insulating, transparent and chemically resistant material, a susceptor (24) provided with disk-shaped cavities (34a-n) for receiving wafers (36a-n) of material to be treated and having an insulating and chemically resistant plate (40) arranged above it, and a diffuser (54) consisting of a plurality of outlet pipes (106a-f) mounted on a cap (52) fixed to an upper opening (50) of the belljar (14).