Abstract Acetic acid was used as a model compound for pyrolysis oil in a hydrodeoxygenation (HDO) study. HDO of acetic acid was performed in a packed bed microreactor. The catalyst was reduced sulfided NiMo/Al2O3. The effects of state of aggregation of acetic acid, temperature, hydrogen partial pressure, liquid flow rate, reactor diameter, and residence time on conversion, yield, space-time consumption, and space-time yield were investigated. External and internal mass transfer and heat transfer resistances were also examined in the microreactor. Temperature was a major factor in HDO of acetic acid. Many consider hydrodeoxygenation as an unattractive process due to high pressure requirements (1050–3000 psig). In this work, attempt has been made to show that HDO of acetic acid can be conducted at atmospheric pressure with a significant conversion achieved. More acetic acid was converted during HDO as temperature was increased at constant pressure of 300 psig. Conversion was much higher for vapor phase acetic acid at atmospheric pressure than liquid phase acetic acid. HDO of gas phase acetic acid in a blank reactor compared to a catalytic HDO showed that thermal decomposition of acetic acid did not occur appreciably. Partial pressure of hydrogen above 240 psig had no effect on the conversion of liquid phase acetic acid. Conversion of vapor phase acetic acid increased as the partial pressure of hydrogen increased from 3 psig to 15 psig. Residence time was 0.06 s for a maximum conversion of liquid phase acetic acid, whereas it was 0.03 s for a maximum conversion of vapor phase acetic acid. The conversion of acetic acid for both liquid and vapor phases decreases significantly as the flow rate of acetic acid increases. As reactor diameter increases beyond 0.8 mm, the conversion reduces significantly. Mass transfer resistance was negligible at the superficial velocity of 2.54 m/s and at an average catalyst particle size of 113 μm. Radial temperature difference in the microreactor was less than 5%.