Abstract:  Esterification of acetic acid with ethanol under microwave heating was carried out by using three types of solid acid catalysts. The results showed that the activity of these catalysts follows the order of Amberlite resin >SO₄²-/ZrO₂ > HZSM-5. These catalysts are also different in their tolerance to water present in the bio-oil. The catalytic activity of SO₄²-/ZrO₂ decreases obviously with increasing water concentration in the reaction mixture, while Amberlite resin still exhibits a high catalytic activity in the presence of water. After catalytic upgrading of crude bio-oil under microwave heating with the cation exchange resin as the catalyst, most of the organic acids are converted into various esters through esterification with ethanol. Microwave heating is more effective than conventional heating and the components of bio-oil are then better upgraded under microwave heating.