The invention relates to a method of processing lignocellulosic biomass comprising: providing lignocellulosic biomass feedstock; pretreating said biomass feedstock by continuous hydrothermal pretreatment in a pressurized reactor; measuring xylan number or lignin number in the output stream of pretreated biomass from the pretreatment reactor; and controlling the pretreatment reactor so as to maintain, in the output stream of pretreated biomass, a pre-determined level of measured xylan number or lignin number. The method allows for continuously controlling the digestibility of a lignocellulosic biomass during the step of pretreatment, and accordingly provides a continuous estimate of the severity of the pretreatment.