A device for treatment of a traveling yarn with a steam-creating medium includes a column of several chambers provided with yarn inlet- and yarn outlet-channels and arranged one over the other, of which one of the chambers is a steam treatment chamber provided with a steam inlet and a condensate outlet with respect to which mixing chambers are respectively operated upstream and downstream thereof, with each such mixing chamber respectively having an air inlet and a condensate outlet. The yarn inlet- and yarn outlet-channels have an opening cross section that, on the one hand, makes possible a pneumatic yarn threading-in and, on the other hand, controls the exchange of air and steam between the chambers such that a steam-air-mixture atmosphere sets in that substantially forecloses the entrance of air into the steam treatment chamber.