An analytical procedure based on alkaline hydrolysis and silylation followed by GC/MS analysis was employed to study the formation of characteristic acidic compounds and the development of a distinctive chromatographic pattern in the course of accelerated ageing tests on Brassicaceae seed oil. On the basis of mass spectra of trimethylsilyl derivatives, the main degradation products were identified as alpha,omega-dicarboxylic, omega-hydroxycarboxylic and dihydroxycarboxylic acids, including 11,12-dihydroxyeicosanoic acid and 13,14-dihydroxydocosanoic acid. The mass spectra of both these compounds are characterised by fragment ions arising from the alpha cleavage of the bond between the two vicinal trimethylsiloxy groups, resulting in fragments at m/z 215 and 345 for 11,12-dihydroxyeicosanoic acid, and at m/z 215 and 373 for 13,14-dihydroxydocosanoic acid. Other significant fragment ion-radicals from rearrangement process at m/z [M - 90](+*), [M - 142](+*), 204 as well as fragment ions at m/z [M - 15](+), [M - 105](+), 217 are present in the mass spectra of both the compounds. The results obtained for reference materials were compared with those relating to archaeological organic materials recovered in Egyptian pottery lamps. The occurrence of the same characteristic degradation products found in the reference materials subjected to accelerated ageing indicates an unambiguous origin for the organic archaeological remains and represents the chemical evidence for the use of oil from seeds of Brassicaceae as illuminant.