This International Standard specifies a method using high-performance size-exclusion chromatography (HPSEC) to determine the contents, as mass fractions, of polymerized triglycerides (PTAGs) in oils and fats which contain at least 3 % (from peak areas) of these polymers. PTAGs (strictly speaking dimeric and oligomeric triglycerides) are formed during the heating of fats and oils, and thus, the method serves to assess the thermal deterioration of frying fats after use.This method is applicable to frying fats and fats and oils that have been thermally treated, provided that the content of PTAGs is at least 3 %. It can also be applied to the determination of p