Table 2 shows contents of TGP found after thermal oxidation at 180 °C for different periods of heating. The tendency to polymeri- sation was clearly dependent on the degree of oil unsaturation, i.e. HOHPSO < HOHPSO/HLSO 95:5 < HOSO < HOSO/HLSO 80:20 < - HOSO/HLSO 50:50 < HOSO/HLSO 25:75 < HLSO. As for the differ- ences between oils, the levels of polymers were very low for the two least unsaturated samples,