The aim of this study was to demonstrate the use of thermogravimetric analysis (TGA) as a potential screening method for monitoring biodiesel production by transesterification of soybean oil with methanol. Soybean oil and commercially available biodiesel were mixed in varying proportions by weight as standards. In addition, mixtures of different biodiesel/soybean oil ratios were also created by periodically interrupting base-catalyzed transesterification of soybean oil with methanol. The mixtures produced by both approaches were analyzed with TGA over a temperature range of 25−500 °C. The results were then compared with analytical data obtained by proton nuclear magnetic resonance spectroscopy (1H NMR spectroscopy), an industry standard for biodiesel quantification. It was found in the TGA experiments that a significant weight loss at ca. 150 °C correlated to the volatilization of biodiesel. The relative weight losses in both sets of mixtures correlated well to the proportion of biodiesel present in the transesterification samples, and the results from both analytical methods were in good agreement (±1.5%). Thus, TGA is a simple, convenient, and economical method for monitoring biodiesel production.