Fuels constitute a group of extreme importance in the economy of any country. Changes in their chemical and physical-chemical properties, such as sediment formation and/or color variation, can occur due to chemical reactions in the moiety and interfere in the products quality, which is not desirable. One of the factors that influence the stability of fuels is the presence of metals, caused by the contact with metallic components in the production, distribution and storage systems, which act as catalysts of these reactions. Since the degradation mechanisms are complex, the study of these reactions is of great importance for a better understanding of the reasons for fuel instability. Thus, this work has the purpose of studying the behavior of the metals lead and mercury in different oxidation states, in the presence of heteroatomic compounds that exist in fuels, using mineral oil as a matrix of hydrocarbons with high purity level. For this research were chosen 1-dodecanethiol and hexanoic acid, which represent the main sulfur compounds - mercaptan - and carboxylic acids present in fuels. The metals were added as the metallic form (Pb0/Hg0) and as oxides (PbO and PbO2/HgO). The study begun with tests performed combining the metals and their oxides with each of these two compounds and with a mixture of both. Afterwards the samples were prepared by adding mineral oil to the combinations above using a sufficient quantity of mercaptan and an acidity that corresponds to that in fuels. These samples were monitored for 1 year (Pb) and 6 months (Hg) in order to observe any alteration in the concentrations added. All control methods used but strong acidity were normalized. Due to its importance, this control method was validated in this work in order to statistically confirm the method s reliability. The results obtained in the validation of the strong acidity method confirmed that the defined conditions were the best. In the case of lead, the results of the initial tests show that this metal reacts with the added compounds in all oxidation states except PbO2, which did not show any alteration when in contact with the acid. In the mineral oil samples alterations in the mercaptan, disulfide and total acidity concentration were noticed in some cases during the observation period. The presence of lead in the solution and the formation of precipitates in most samples were also evidenced. Evaluating each sample individually, the redox processes that occurred in each one could be observed and we concluded that the oxidation state of lead directly influences the reaction s kinetics. Among the mineral oil samples the most critical case occurs when Pb4+ contacts the mercaptan and hexanoic acid in oil, because in these conditions the mercaptan oxidizes and forms the correspondent sulfonic acid. In this case, the presence of strong acidity was evidenced by the initial potential shift in the positive direction and by other alterations that occurred in solution. In the case of mercury it could be concluded that in all preliminary tests performed, a reaction occurred both with metallic mercury (Hg0) and with the oxide (HgO). The only case that did not react in the period studied was that of Hg0 with the acid. In the mineral oil samples the presence of mercury was detected in all samples due to the solubility of mercury compounds in this moiety. The alterations that occurred in the case of Hg2+ indicate that this ion reacts with the added compounds, including mineral oil. Based on these results it was possible to conclude that the presence of these metals, in all of their oxidation states, causes them to react with the heteroatomic compounds of sulfur and oxygen that are present in fuels, forming degradation products which affect the quality of the product destined to consumption.