An oxidation furnace for the oxidative treatment of fibers, in particular for producing carbon fibers, with a housing which is gas-tight, apart from passage areas for the carbon fibers, and a process chamber located in the interior of the housing. Hot air can be blown into the process chamber by at least one air inlet device. Deflecting rollers flanking the process chamber guide the fibers arranged side by side in the form of a carpet through the process chamber in a serpentine manner, wherein each fiber carpet spans a plane between opposite deflecting rollers. The air inlet device allows hot air to be diverted to the side of the deflecting rollers facing away from the process chamber such that hot air flows over the respective deflecting roller and the fibers before it enters the process chamber.