This research focuses on the synthesis and characterization of mesoporous niobosilicate molecular sieves and their catalytic activity in biodiesel production by transesterification of sunflower oil with methanol. Catalysts were prepared by two procedures: impregnation of a MCM-41 silica with different amounts of niobium oxalate and subsequent calcination, and structural incorporation of Nb into a MCM-41 silica during the synthesis step. Characterization techniques such as XRD, XPS, TEM. NH3-TPD and N-2 sorption have been employed to characterize the synthesized catalysts. The biodiesel yield increases with the catalyst acidity, attaining a value of 95% with a 7.5 wt% of a MCM-41 silica impregnated with a 8% of Nb2O5, at 200 degrees C, after 4 h of reaction and a methanol/oil molar ratio of 12. The potential of this family of catalysts to treat low-grade oils has been demonstrated by increasing the acidity of the sunflower oil by adding oleic acid (1.1 wt%) and water (0.2 wt%) to the reaction mixture, since the biodiesel yield is maintained close to 80%. Moreover, the catalyst reutilization has been demonstrated during five catalytic runs by employing a low-grade oil, with no leaching of the active phase. (C) 2011 Elsevier B.V. All rights reserved.