Synergy in co-liquefaction of oil shale and willow in supercritical water

•A synergy occurs in the yields of oil shale and willow co-liquefaction products.•The synergies are proportional with product of the shares of the components (xAxB) in the blends.•The coefficient of proportionality, synergy factor, is the highest for the benzene soluble products.•The synergy in the yield of benzene solubles is caused mainly by an increase in the yield of high polar heterocompounds.