A series of physicochemical studies performed on Brazilian commercial Bx (0%, 7%, 20%, and 100% soybean/ tallow biodiesel) mixtures in SIO and 5500 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. 5500 blends are more acidic than SIO blends, in accordance with the higher water content of the former. Rancimat accelerated oxidative stability tests showed that, as expected, 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 the studied band. On the other hand, Raman spectroscopy represents a very suitable spectroscopic probe for unsaturations in the fatty acids chains of biodiesel. Since FTIR-HATR and Raman spectroscopies do not require any sample preparation, are fast and quite low cost techniques, and cause low impact to the environment, further attention must be paid to their use in the analysis of biodiesel-containing fuels degradation.