AbstractSunflower seed oils were compared with a commercial oil based upon the oils' tendencies to oxidize after various periods of heating and cooking. Hydrogenated and unhydrogenated sunflower oils and a commerical shortening obtained from a fast‐food establishment were used to deep fry 8 lb of raw potatoes daily for six 8‐hr days. Samples of oils were taken daily and active oxygen method values determined. A plot of the log of active oxygen method values vs. the time gave a straight line, the slope of which reflects the oxidizability of the oil. The partially hydrogenated northern sunflower oil was much less prone to oxidation after use than the commercial shortening even with its lower initial active oxygen method value.