AbstractThis article reviews old and recent literature concerning the hypothesis that the Diels‐Alder reaction is the dominant reaction in the thermal polymerization of vegetable oil. Both triacylglycerol oils and methyl esters are used to show that this mechanism is unlikely to be a significant contributor en route to the reaction products. The products formed under a range of different reaction conditions are reviewed, including dimer acid, where Diels‐Alder chemistry is not claimed, to the polymerization of mono‐unsaturated methyl oleate in the presence of oxygen. In these, and even in the system where Diels‐Alder chemistry is claimed, the polymerization of conjugated methyl linoleate, six‐membered ring structures are not found in levels detectable by 13C NMR. Since such structures are formed by a Diels‐Alder reaction, it is unlikely that any more than 5% of the reaction could be formed by that method. Other factors against the Diels‐Alder contribution to this process, such as conformation and electron withdrawing ability of the substrates are also discussed.