The aim of the present work was to verify the influence caused by the contamination or the addition of residual oil in the blends commercialized in Brazil [2% (v/v) of biodiesel in petrodiesel B2] in physical-chemistry parameters usually used to monitor the fuel quality, such as viscosity, flash point, distillation curve. specific gravity. and cetane index. To carry out the experiments, a set of samples simulating a system of fuel adulteration, mixing together biodiesel and residual oil, was prepared in concentrations varying front 0.5 to 25% (w/w). Then, these samples were submitted to the physical-chemistry assays cited before. The specific gravity presented itself as the most sensitive to adulteration. It detected the adulteration in concentration values of residual oil above 10% (w/w). Afterward, a calibration model was built using a multivariate calibration tool, partial least-squares (PLS), applied to attenuated total reflection/Fourier transformed infrared spectroscopy (ATR/FTIR) data to quantify the residual oil present in each sample. The PLS technique was shown to be very efficient in the determination of adulteration of B2 with residual oil from 0.5 to 25% (w/w).