Abstract: An internal gas generator in combination with a standard heat treat furnace is provided for producing either an endothermic product gas or a purge gas. The generator includes a reaction tube containing alternating packed beds of a highly active catalyst and inert heat transfer particulates. The reaction tube is surrounded by an elongated tubular heating element which heats the reaction tube, the combination producing an acceptable product gas in a small arrangement suitable for retrofit applications to existing furnaces. A flanged mounting permits the reaction tube to be easily removed for standard replacement in the event of catalyst poisoning and the like.