Installation for treatment of wafers in a reactor. To that end a series of wafers is placed in a wafer rack and fed into the reactor. Transport into and out of the reactor, which is sited in an enclosed chamber, takes place with the aid of conveyor means. The wafers are transferred from the wafer rack to one or more cassettes. During this operation the wafer rack is always in the vertical position, that is to say the wafers are horizontal. The same preferably also applies to the cassettes, so that the wafers remain horizontal throughout the entire process.