A simple reactive condensation technique was developed to decrease the concentration of reactive species in the oily phase of two-phase pyrolysis oil as a means to increase the storage stability, heating value, and overall quality of the oil. Bio-oil vapor was esterified from the reaction of in situ organic acids with ethanol during condensation resulting in the production of water and esters. The research compared the quality of slow pyrolysis-produced bio-oils collected by condensing pyrolysis vapor using atomized ethanol (EtOH) at various weight hourly space velocity (WHSV). The resultant bio-oil exhibited two phases at room temperature, which were separated to obtain an oily and an aqueous phase. WHSV was varied from 8.3 to 33 (kg h−1 biomass per kg h−1 ethanol), which produced bio-oils with ethanol content ranging from 7.3 to 23.2 wt % of total oil. Increasing EtOH from 0 to 23% (w/w) decreased water content and viscosity in the oily phase by 16% and 56%, respectively, while also increasing pH from 2...