A series of catalysts containing 5-30 wt% of 12-tungstophosphoric acid (TPA) supported on niobia were prepared and their catalytic activity was evaluated for esterification of free fatty acids with methanol. The catalysts were characterized by FT-infrared spectroscopy, X-ray diffraction and temperature programmed desorption of ammonia. The esterification activity depends on the content of TPA on niobia; the catalyst with 25 wt% of TPA exhibited highest activity. The best catalyst was subjected to different calcination temperatures to study the structural changes. We observed that esterification activity depends upon the structural variations of the catalyst. The effects of reaction variables such as catalyst loading, methanol to acid ratio, reaction time and temperature on the conversion of fatty acid were studied. The esterification activity of the catalysts is correlated with the characteristics of the catalysts. (C)c 2009 Published by Elsevier B.V.