For producing fuel by the liquefaction of wood, it would be reasonable to use a raw material in the form of large granules instead of sawdust or powder. In this paper, a simple mathematical model, for the solvolysis of wood dowels of various diameters in acidified phenol is described. The modelling is based on a dual diffusion reaction. In order to determine the kinetic constants of the reaction, when the rate is entirely controlled by the chemical reaction, experiments have been carried out on wood powder, well dispersed in the solvent. For the larger pieces of wood, the dissolution appears as a stepwise process. An explanation of this process is given, based on morphological changes in the wood matrix during solvolysis.