Complex materials as fuel oils are characterized by physical and chemical parameters. This paper deals with the determination of the activation energy of three kinds of Brazilian fuel oils. As it is possible to establish a direct relation between ignition delay and activation energy, these kinetic parameters can be used to characterize these oils regarding the combustion quality of the fuel. Activation energies were determined as a function of conversion degree ((x) and temperature by model-free kinetics. The samples, here named A, B, and C, were supplied by Petrobras-Cenpes, and they were characterized by thermogravimetry (TG) and differential thermal analysis (DTA). Transient experiments were performed from room temperature up to 600 degrees C at five heating rates (2.5, 5.0, 10.0, 15.0, and 20.0 degrees C min(-1)) in an atmosphere of synthetic air and samples of about 20.0 mg were used. In all experiments, three distinct decomposition regions were observed and identified as low-temperature oxidation (LTO), fuel deposition (FD), and high-temperature oxidation (HTO). In order to compare these oils, we considered the medium value of the activation energy obtained between conversions of 1% and 90% only for the LTO region. The results are as follows: sample A = 43.8 kJ mol(-1), sample B = 57.2 kJ mol(-1), and sample C = 61.8 W mol(-1), showing that the sample A is the best option among the samples studied in this work and that the activation energy is a suitable parameter for this purpose.