[en] Highlights: • Pyrolysis oil upgrading under supercritical water, tetralin and 1-MN above 400 °C. • The highest feedstock conversion of 90 wt.% with SCW and tetralin. • Produced heavy oil with relatively low oxygen content (∼15%) with SCW and tetralin. • 1-MN-tetralin (mass ratio 80%:20%) gave the same product conversion as the runs with tetralin ∼90 wt.%. - Abstract: The upgrading of pyrolysis oil to bio-fuel in high conversion was investigated using supercritical water (SCW) above 400 °C with the 75 mL reactor for 1 h. Tetralin and 1-methylnaphtalene provided a baseline product conversion as a good hydrogen donor and non-hydrogen donor (eventhough with much lower mass to solvent mass ratio). Experiments under SCW indicate that conversions increased with increasing pyrolysis oil to water mass ratio at high water ratio, these giving the highest products conversion of 91 wt.%, with 28 wt.% heavy oil recovered, 23 wt.% gas yield, 27 wt.% water generated and approximately 13−14 wt.% of light oil produced. Tetralin runs gave similar product conversion of 90 wt.% but with lower sample to solvent mass ratio (1:2), and 1-methylnaphtalene yielded much lower conversion 70 wt.% using the same sample to solvent mass ratio. A combination of 1-methylnaphtalene/tetralin gave the same feedstock conversion (90 wt.%), suggesting that only a small amount of hydrogen needed to produce high conversion. The oxygen contents of the heavy oil recovered were 16% for SCW, with H/C atomic ratios of 1.1. The heating value of upgraded oil was increased to 33 MJ/kg compared to 18.5 MJ/kg of initial pyrolysis oil. The heavy oil with similar product conversion (90 wt.%) using tetralin and much lower solvent to oil ratios had slightly lower oxygen content (14%), and the H content was increased from 7.0 wt.% to 7.3 wt.%.