The invention relates to a process for treating lignocellulosic biomass by means of an acidic liquor in order to produce “second-generation” (2G) sugary liquors. These sugary liquors may be used to produce other products via a biochemical pathway (e.g. alcohols such as ethanol, butanol or other molecules, for example solvents such as acetone, etc.). This process comprises analysis of the liquid streams withdrawn during the process and adjustment of the composition of the impregnation liquor so as to keep the acidic power of said streams constant.