Raw and modified forms of clinoptilolite were used as catalyst to test their activities in the esterification of glycerol with oleic acid. Several chemical treatments were conducted to obtain a suitable catalyst for the selective formation of monoester while suppressing the formation of di- and tri-esters. Raw clinoptilolite was modified to obtain modified clinoptilolites with various Si/Al ratio by two different methods; passivation with tetraethylorthosilicate (TEOS) and dealumination with HCl which both increase the Si/Al ratio. Catalysts were characterized by X-ray diffraction (XRD), Fourier transform-infrared (FT-IR), X-ray photoelectron (XPS) spectroscopy, thermogravimetric/differential thermal analysis (TG/DTA), and N(2) gas adsorption-desorption techniques. The results of the catalytic tests indicate that clinoptilolite can catalyze the esterification of glycerol with oleic acid efficiently. The highest acid conversion and selectivity to monoolein were obtained in the case of the acid treated clinoptilolite. This catalyst proves to be a highly efficient catalyst giving a very high selectivity to monoolein similar to 63% with the conversion of oleic acid similar to 80% at 443 K. (C) 2009 Elsevier Inc. All rights reserved.