The molecular structure (carbon chain length, degree of unsaturation and branching of the chains) and the presence of contaminants in a biodiesel have a critical influence on their lubricant behaviour. Ball-on-disc tri-bological tests were performed with two different standard reference materials for biodiesel - NIST SRM 2772 (soybean) and NIST SRM 2773 (animal fat). It is demonstrated that the Stribeck curves, the plots of coefficient of friction (p) vs. Stribeck parameter S (the product of viscosity with speed divided by the normal contact load), fit to a general polynomial equation of the type mu = mu(0) (1- K-BL x S + K-EHL X S-n). Constants K-BL and K-EHL reveal the contributions of the boundary lubrication (BL) and the elastohydrodynamic (EHL) phenomena, respectively. The animal fat biodiesel is the most effective fuel as it shows the highest KBL and the lowest K-EHL values leading to the most stable coefficient of friction regarding speed variation. This behaviour is attributed to the higher content of contaminants, namely mono- and di-glycerides, and the presence of sulphur in the animal fat biodiesel. (C) 2013 Elsevier B.V. All rights reserved.