A molecular absorption method is proposed for the determination of phosphorus in biodiesel. The samples are mineralized using an ashing procedure at 550 degrees C followed by dissolution of the residue. The analytical procedure is based on the formation of a blue molybdenum complex. 1-amino-2-naphthol-4-sulfonic acid is used as reducing agent. The method was applied to biodiesel samples prepared from soy, canola and sunflower oils and from bovine fat. The limit of detection is 0.57 mg P kg(-1) and the limit of quantification is 1.7 mg P kg(-1). The observed mean relative standard deviation is about 5%. The simplicity of the procedure added to its precision, accuracy and low cost suggest that it is an excellent option for the determination of phosphorus in biodiesel. (C) 2011 Elsevier Ltd. All rights reserved.