The aim of this experiment was to investigate the storage stability of a linseed-sunflower meal co-extrudate, which was produced as a functional additive to improve the fatty acid composition of animal feed. Since a high content of α-linolenic acid causes low oxidative stability of this product, antioxidants, vitamin E (135 mg· 100 g–1) and carvacrol (200 mg· 100 g–1), were added to the co-extrudate in order to prolong its shelf life. The ratio of linseed to sunflower meal in the extruded mixture was 50:50. The produced co-extrudate was stored in a climate chamber at 63 ± 2°C in order to examine changes during storage. The following chemical parameters were determined to detect changes in the fat phase of the co-extrudate: fatty acid composition, peroxide value and free fatty acid content. The samples were also monitored for their microbiological status. Although changes in the peroxide value as well as in the free fatty acid composition were significant, changes in the fatty acid content were minimal. Carvacrol gave excellent results in suppressing microorganism development, while it proved to be less effective as an antioxidant. Vitamin E, on the other hand, showed very good antioxidant activity, but had no significant effect on the growth and development of microorganisms. These two substances did not show any synergistic oxidative effect.