Different ratios of phosphotungestic acid supported on silica gel were prepared by an impregnation method with PWA loadings ranging from 1 to 30% w/w and calcined at 350 and 500 degrees C for 4h in a static air atmosphere. The catalysts were characterized by thermogravimety (TG), differential thermal analysis (DTA), X-ray diffraction, FT-IR spectroscopy and N-2 adsorption measurements. The surface acidity and basicity of the catalyst were investigated by the dehydration-dehydrogenation of isopropanol and the adsorption of pyridine (PY) and 2,6-dimethyl pyridine (DMPY). The gas-phase estrification of acetic acid with ethanol was carried out at 185 degrees C in a conventional fixed-bed reactor at 1 atm using air as carrier gas. The results clearly revealed that the catalyst containing 10% w/w PWA/SiO2 is the most active and delivers reaction selectively to ester with 85% yield. The Bronsted acid site resulting from hydroxylation of tungsten oxide plays the main role in the formation of ester. (c) 2007 Society of Chemical Industry.