One of the most important properties measured for liquid fuels is the volatility, usually as expressed by the distillation curve. In previous work, we introduced the composition-explicit or advanced distillation curve (ADC) metrology, which we have applied to a wide variety of liquid fuels, including biodiesel fuels. Application of this method to typical biodiesel fuels requires the addition of an inert gas purge in the apparatus to achieve acceptable repeatability. Despite this precaution, there is clear evidence of thermal decomposition or polymerization at high distillation temperatures. To address this, a low-pressure version of the ADC was introduced. In this brief paper, we apply the low-pressure ADC to a commercial soy-based biodiesel fuel and discuss the volatility differences compared to measurements at atmospheric pressure. We show that, by reducing the overall boiling temperatures, the reduced-pressure distillation measurement avoided thermally induced decomposition and polymerization that can occur late in the distillation curve measurement. Analysis by gas chromatography (with flame ionization detection and mass spectrometry) confirmed the sample decomposition at atmospheric pressure. Furthermore, this analytical approach provided additional insight into the decomposition process and the resulting products at lower pressures.