Studies on bio-oil production by thermochemical liquefaction are usually focused on the optimization of temperature, particle size, and residence time to obtain high yields, and the effects of these process parameters on biomass liquefaction are relatively well understood. However, there is a lack of agreement in previous studies on the effect of biomass-to-solvent (BS) ratio, and contradictory results have been reported. In this study, the effect of BS ratio was examined in detail using macroalgae (Saccharina japonica) as the biomass and supercritical ethanol as the liquefaction solvent at 400 degrees C under N-2 environment. The BS ratio was varied by changing the amount of either the biomass or the solvent, and the yields of the liquefaction products (i.e., bio-oil, gas, and solid residue) and the bio-oil quality were compared. When the BS ratio was increased from 0.10 to 0.17 by changing the amount of ethanol, the bio-oil yield decreased significantly from 79.5 to 37.8 wt%, whereas for the same increase in BS ratio effected by changing the amount of biomass, the bio-oil yield decreased less significantly from 79.5 to 60.0 wt%. This was attributed to the combined effect of BS ratio and reaction pressure on the liquefaction reaction. (C) 2016 Elsevier B.V. All rights reserved.