Abstract: Disclosed is a process for the thermal-mechanical pretreatment of biomass. The process includes subjecting a biomass feedstock including fibers containing cellulose, hemicellulose and lignin, to thermal reaction under conditions exceeding atmospheric pressure, at a temperature exceeding ambient temperature, at a predetermined moisture content and for a predetermined amount of time. Subsequently, the pressure of said thermal reaction is reduced under conditions resulting in explosive decompression of said biomass. The decompressed biomass is then subjected to axial shear forces to mechanically reduce the size of the fibers of the biomass to obtain treated biomass. The resultant treated biomass has a high level of enzymatic digestability and a low concentration of degradation products.