Abstract: A regenerative thermal oxidizer for oxidizing a contaminant in an air or other gas stream includes a heat transfer material which heats the incoming air and collects heat from the exiting air. The direction of flow of the air through the oxidizer is periodically reverse for the regenerative heat exchange. When flow is reversed, a quantity of untreated air in the oxidizer which would otherwise be discharged to the atmosphere is collected in a purge chamber which surrounds the lower portion of the discharge stack. The untreated air enters the bottom of the chamber and the air already in the chamber is forced out from vent holes in the top of the chamber into the stack. The untreated air in the chamber is fed back to the oxidizer through the inlet of the blower feeding the oxidizer while clean air is drawn into the chamber from the stack through the vent holes.