A series of physicochemical studies performed on Brazilian commercial Bx (0 per cent, 7 per cent, 20 per cent, and 100 per cent soybean/tallow biodiesel) mixtures in S10 and S500 oil diesel, as well as the performance of two rapid and still underexplored techniques, namely, FTIR-HATR and Raman spectroscopies, to evaluate the hydrolysis and oxidative stability of these blends are reported. The addition of biodiesel to diesel affects negatively the aging resistance of the resulting blends. S500 blends are more acidic then S10 blends, in accordance with the higher water content of the former. Rancimat accelerated oxidative stability tests showed that the induction times of B7 and B20 samples are greater than that of B100, independent of the sulfur content of the diesel. The practical use of FTIR-HATR to characterize the mixtures degradation stage is conditioned by the fact that there are two chemical contributions for each of the studied bands. On the other hand, Raman spectroscopy represents a very suitable spectroscopic technique to detect the presence of unsaturations in the fatty acids chains of biodiesel. Since FTIR-HATR and Raman spectroscopies do not require sample preparation, are fast and quite low cost techniques, and cause low impact to the environment, further attention may be paid to them.