For the first time, a mathematical model has been proposed to describe the influence of blending ratio on the synergy in co-pyrolysis. The model is based on the stability of a new cross-compound A(n)B formed between the pyrolysis products of the blend components. A new characteristic, named synergy factor (delta), has been introduced to express the synergy formula. The value of d and synergy in oil yield are positive when A(n)B is volatile or soluble in the solvents applied for oil separation. As an example, the model deduced was proved in the mathematical processing of earlier published experimental results on the co-pyrolysis of oil shale and pine wood in supercritical water at 380 degrees C during 4 hours. The values of d were estimated for the subsequent distribution of the pyrolysate into water extract (including ether soluble and insoluble extracts), water insoluble oil (including benzene and acetone extracts), solid residue, and gas and pyrogenetic water.