We report a new approach to identify swill-cooked oils that are recycled from tainted food and livestock waste from commercial vegetable and animal oils by means of carbon isotope values and relative abundance of fatty acids. We test this method using 40 cooking oil samples of different types with known sources. We found significant differences in both total organic carbon isotope as well as compound-specific isotope values and fatty acid C-14/C-18 ratios between commercial vegetable oils refined from C-3 plants (from -35.7 to -27.0 parts per thousand and from 0 to 0.15) and animal oils (from -28.3 to -14.3 parts per thousand and from 0.1 to 0.6). Tested swill-cooked oils, which were generally refined by mixing with animal waste illegally, fall into a narrow delta C-13/fatty acid ratio distribution: from -25.9 to -24.1 parts per thousand and from 0.1 to 0.2. Our data demonstrate that the index of a cross-plotting between fatty acid delta C-13 values and C-14/C-18 ratios can be used to distinguish clean commercial cooking oils from illegal swill-cooked oils.