Easy sonic-spray ionization mass spectrometry (EASI-MS) allows for direct and fast MS analysis of samples in ambient conditions with little or no sample preparation, therefore offering unprecedented simplicity, speed, and ease of use EASI-MS has been shown to access the quality, type, and adulteration of biofuels and vegetable oils. Herein, EASI-MS is shown to quantitate and monitor the quality of soybean biodiesel/petrodiesel (Bn) blends. For adulteration, admixture of the parent oil has been tested and nearly instantaneous and direct EASI(+)-MS detection of as little as 1% (w/w) of soybean oil in biodiesel/petrodiesel blends was achieved Linear analytical curves (r > 0.98) were also obtained for the quantitation of Bn blends, and the EASI(+)-MS quantitation results were compared to those obtained by nuclear magnetic resonance (NMR) spectroscopy and mid infrared (IR) spectroscopy.