Deep-fat frying process is a commonly used procedure for food preparation, and the oxidative stability is an important quality issue for frying oils. In this study, the effect of citrus peel extracts (orange, lemon, mandarin) and BHT (butylated hydroxytoluene; a synthetic antioxidant) on oxidative stability of sunflower oil during the deep-fat frying process was investigated. For this purpose; classic chemical wet methods (free fatty acid, peroxide, p-anisidine, conjugated dienes) and a spectroscopic method (Electron Paramagnetic Resonance [EPR] spectroscopy, spin trapping technique) were used. According to both chemical data and EPR spin trapping results, the antioxidant effects of citrus peel extracts were found to be comparable to BHT. Moreover, the antioxidant effect of lemon peel extract was higher than other citrus peel extracts. EPR spin trapping technique can be used as an earlier and more accurate detection method in determining lipid oxidation during repetitive deep-fat frying. Practical applications: Citrus peels, which are among the by-products of the fruit juice industry, have antioxidant properties. For this reason, citrus peels have the potential to improve the safety, quality, and nutritional value of industrial edible oils. In addition, waste can be reduced and value-added products can be produced by the utilization of citrus peels. EPR spectroscopy is the only technique in which lipid radicals, which are the most important indicators of lipid oxidation in industrial edible oils, can be directly and precisely defined. However, today mostly classical methods are used to measure oil quality. The more sensitive detection of oxidation in industrial edible oils may give us a clearer knowledge about the efficiency of the antioxidant source and the conditions under which they can be used.