Oxidative stability is closely dependent on the degree of unsaturation of the oil and also on the presence of antioxidants, which are likewise affected by the temperature. This analysis measures the total volatile carbonyl compounds, which are secondary products of lipid oxidation, and it also evaluates the oil’s resistance to oxidation. There was a significant reduction in the induction period during the heating time of soybean oil, with or without the addition of antioxidants. This fact indicates that the antioxidants tested were unable to remain stable after20 h of heating. The added antioxidants greatly influenced the oxidative stability of the heated soybean oil. Nevertheless, TBHQ efficiency was markedly weaker than the rosemary extract when the thermoxidation process was employed. Rosemary extract was more efficient than TBHQ, the synthetic antioxidant, in preventing the decrease of oxidative stability, and it produced a protective effect. Its stability values were significantly higher than those that resulted from the TBHQ treatment in all the analyzed periods. Moreover, it was found that there was no synergistic effect between TBHQ and rosemary extract, because the samples, RE and RE + TBHQ, showed no significant difference after 10 or 20 h of heating.