The fractionation of wood consists of the separation of the fibrous material into its constitutive polymers (i.e. extractives, hemicellulose, cellulose and lignin) in a sequential and selective way. The thermochemical or biochemical upgrading of each fraction leads to the production of common and fine chemicals. The first step of the fractionation process is the pretreatment which permits the solubilization of extractives and hemicellulose. The residue thus obtained is more accessible to chemicals and enzymes. The goal of this paper is to prove that a thermomechanical pretreatment in aqueous phase (without adding any chemical) may solubilize, in a relatively selective way, the hemicellulose. This process consists of the following steps: 1) Steeping of aspen sawdust (0·25–0.·50 mm) in water at 7% consistency for 16 h; 2) Cooking at 165–235°C; 3) Rapid discharge (i.e explosion) at δp=6·9 MPa; 4) Quenching. The extent (i.e conversion) and selectivity (i.e. hemicellulose removal vs delignification) of wood solubilization have been studied as a function of cooking temperature in a batch reactor system.