Biodiesel content on biodiesel/diesel blends is obtained by determining the band intensity of C=O bond in the fatty acid methyl esters (FAME) of the biodiesel by mid-infrared spectroscopy (reference method: EN 14078, 2014). The potential for biodiesel/diesel blends to be adulterated with vegetable oils constitutes a limitation of the reference methods capacity to accurately quantify the biodiesel content in these blends since vegetable oils, composed primarily of triacylglycerols, also contain the C=O bond. This study employed normal-phase high-performance liquid chromatography with a refractive index detector (NP-HPLC-RI) to quantify the biodiesel in biodiesel/diesel blends and detect potential adulterations of these blends with vegetable oils. Two calibration curves (4 to 12% vol and 5 to 30% vol) were plotted for the biodiesel quantification based on which 12 verification samples were analyzed (samples prepared at different concentrations from the calibration curves), as well as 20 samples of commercial diesel, acquired at gas stations in the southeast region of Brazil. The NP-HPLC-RI method presented good analytical performance in terms of linearity, limit of detection (LOD), limit of quantification (LOQ), precision (repeatability), accuracy (recovery), and robustness. Linearity was determined by the coefficient of determination (R-2) for concentrations of biodiesel and vegetable oil in diesel varying from 4 to 12% vol (R-2 = 0.9924 and R-2 = 0.9950, respectively) and from 5 to 30% vol (R-2 = 0.9968 and R-2 = 0.9962, respectively). The LOD and LOQ for the quantification of the biodiesel were 0.08 and 0.23% vol, while for the quantification of soybean oil, these values were 0.07 and 0.21% vol, respectively. The recovery values varied from 97.7 +/- 1.8% to 107.1 +/- 4.1%, indicating good accuracy, and the method proved robust when the temperature was changed from 40 to 35 degrees C. The paired sample t-test showed the nonexistence of significant differences between the proposed and reference methods (with 95% confidence), indicating the capacity of NP-HPLC-RI to detect and quantify biodiesel and vegetable oil adulterants in samples of diesel both rapidly and effectively, thereby demonstrating its importance for the quality control of this fuel since the current methodology (EN 14078) used in several European Union countries, as well as in Brazil and Argentina, cannot identify this kind of adulteration and cannot accurately analyze the biodiesel content in biodiesel/petrodiesel blends.