Cyclic fatty acid monomers (CFAM) are formed at low levels in edible oils during thermal processing operations such as frying or refining, and inevitably become part of the diet. These proatherogenic agents may increase the levels of oxidative stress markers, and induce hepatomegaly and steatosis. However, the kinetics involved in their formation is not well known. The objective of the present study was to evaluate the effects of cis and trans isomers on cyclization reactions involved in the thermal transformation of alpha-linolenic acid (ALA). Geometrical isomers of ALA were obtained from all-cis ALA by nitric acid treatment. Mono-trans isomers were concentrated using silver nitrate-silica gel chromatography. All-cis ALA, isomerized ALA, and a fraction at 85% mono-trans isomers were heat treated at 275 degrees C in hexadecane for periods up to 24 hours, and the formation of geometrical isomers and CFAM was monitored by GC. The results show that mono-trans isomers at carbon 9 and carbon 15 form CFAM at an accelerated rate, compared to the corresponding cis isomers, resulting in the formation of higher levels of CFAM over shorter time periods. The validation of the kinetic model was performed by solving simultaneously and nonlinearly fitting the system of coupled differential equations with experimental data. Good agreement was found between the experimental data and the predicted values. This work suggests that the use of polyunsaturated vegetable oils over extended periods for thermal processing of food may result in the formation of CFAM, in particular, if mono-trans isomers are present in the oil.