An analytical method for the analysis of free glycerin and total glycerin in biodiesel/diesel blends is presented. The total glycerin is related to the amount of nonconverted glycerides present in the biodiesel, while the free glycerin is related to its purification steps. The method is based on the conversion of mono-, di-, and triacylglycerides in methyl esters and glycerin, and then the glycerin is extracted and quantified with a standard chemical procedure. It is possible to measure the total and free glycerin with very high repeatability and reproducibility. The limits of detection and quantification for the total glycerin analysis are 3.1 x 10(-5) and 9.1 x 10(-5) wt %, respectively, making it possible to determine this parameter in biodiesel/diesel blends containing a low amount of biodiesel of good quality.