Formation of trans fatty acids under the influence of heat was studied in linseed, pumpkin, rape, soybean and sunflower oils and seeds. The samples were exposed to temperatures between 423 and 573 K for 30 to 180 min. Fatty acids were analysed by gas chromatography using a Flame Ionization Detector. In the case of linseed and linseed oil the results were examined by the application of a GC/MS analysis. The ionisation mode was electron impact. At 473 K slight formation of trans fatty acids could be observed in seeds due to the exposure to high temperatures, while in oils small traces were only found at 523 K and after 120 min. In seeds the share of trans fatty acids on the fatty acid spectrum amounted approximately to five times that of oils. It may be argued that the enhancement of cis-trans isomerisation in seeds might be caused by the presence of thiyl radicals in intact organic matter.