Palm oil, soybean oil and partially hydrogenated soybean oil were used to define the repeatability of oil degradation under thermoxidation in the absence of food and during the frying of potatoes. Total polar compounds and their constituent polymeric triglycerides, oxidized monomeric triglycerides and diglycerides were analyzed. The oils were thermoxidized under standard conditions in a Rancimat apparatus at 180°C for 25 hours. Discontinuous frying experiments were carried out in triplicate in three apparently identical domestic fryers. Five batches of potatoes were fried per day in each fryer. Oils were heated at 180°C in periods of 5 h per day during 5 consecutive days so that the total heating period was 25 h. Repeatability was high in the thermoxidation assays and rather low in the frying experiments. Significant differences between the oils were found in the thermoxidation experiments, while no significant differences were found in the frying assays due to large standard deviations. The participation of variables related to the fryer performance which are difficult to control, was deduced. Among them, differences in the heating/cooling cycles at high temperature might be important. The contribution of fryer performance to the decreased repeatability was confirmed in further frying experiments with palm oil. Results showed that repeatability for the same fryer was higher than that obtained when different fryers were considered.Se estudia la repetibilidad de los resultados obtenidos en el calentamiento de los aceites y grasas a temperatura elevada utilizando aceite de palma, aceite de soja y aceite de soja parcialmente hidrogenado. Partiendo de la repetibilidad de las determinaciones analíticas utilizadas para la evaluación de la degradación de los aceites y grasas a elevada temperatura, el análisis de compuestos polares y de su distribución en polímeros de triglicéridos, triglicéridos monómeros oxidados y diglicéridos, se analizan las desviaciones obtenidas durante el tratamiento termoxidativo en condiciones estándar (Rancimat a 180 °C durante 25 horas) y durante la fritura de patatas en freidoras domésticas en condiciones bien controladas. Los resultados indican la existencia de diferencias significativas en el comportamiento de los aceites en el tratamiento termoxidativo en ausencia de alimentos aunque no fue posible encontrar diferencias significativas en la fritura discontinua cuando se realizan los triplicados en distintas freidoras, debido a las elevadas desviaciones obtenidas entre ensayos. En un posterior ensayo con aceite de palma utilizando la misma freidora para los tres ensayos de frituras se obtuvieron menores desviaciones. Los resultados indican la existencia de variables de difícil control entre las que se apuntan diferencias de temperatura media debido a las diferencias en los ciclos de calentamiento/enfriamiento a temperatura elevada.This research work was supported by Junta de Andalucía and MCYT (Project AGL 2004-00148). Eliana Rodrigues from the Instituto Nacional de Controle de Qualidade em Saúde/ FIOCRUZ was supported by a fellowship from Coordenação de Aperfeiçoamento de Pessoal de Nível Superior do Brasil (CAPES).Peer reviewed