A green and simple visual method is proposed for the determination of the acid number of biodiesel. The method was compared with that of the AOCS Cd 3d-63. The sample is dissolved in a 1:1 (v/v) ethanol: water mixture and phenolphthalein is added. This method replaces the use of the 1:1 (v/v) toluene: isopropanol mixture as solvent. The titration is performed with a 0.02 mol L (1) NaOH solution in water, avoiding the 0.1 mol L (1) KOH solution in isopropanol. A statistical comparison using the Student's t and the Snedecor F tests showed that the proposed green method offers equivalent results to AOCS Cd 3d-63, with a little better precision. (C) 2011 Elsevier Ltd. All rights reserved.