The aim of this study was to investigate the authenticity of cold pressed oils on the ground of content of trans isomers of fatty acids and 3,5- stigmastadiene by gas chromatography method. 30 accessible on the Warsaw market trade oils: cold pressed from seeds and for comparisons refined and olive oils: extra virgin, sansa, pomace, light, extra light were the investigative material. It was shown, that the determination of authenticity of cold pressed oils was possible to execute on the ground of the analyses of trans isomers of fatty acids and the determination of 3,5 stigmastadiene. Results of these analyses can be the criterion of adulterations cold pressed oils by refined oils of the same kind. The fatty acid composition of analysed oils was characteristic for individual species of oils. In the case of three oils from seeds and in six olive oils trans isomers of fatty acids on the level 0,1-0,6% were detected. Only in peanut oil 3,5-stigmastadiene was not detected. Extra virgin olive oils had low content of 3,5-stigmastadiene (0,13 to 1,12 mg/kg). Particularly large content of this compound (6,54 to 52,94 mg/kg) was detected in the grapeseed oils, sunflower cold pressed oil and olive oils of type sansa, pomace, light, and extra light. Three oils from among eleven from the seeds which were declared as cold pressed could be subjected by refinement or become falsified by confusion by refined oils. In both the presence of trans isomers of fatty acids and 3,5- stigmastadiene was detected.