A process for the pretreatment of lignocellulosic biomass is disclosed. The process is intended for use in connection with biomass to ethanol processes and is directed in particular to an economical removal of inhibitory compounds generated in biomass pretreatment, which are inhibitory to downstream hydrolysis and fermentation steps. The process includes the steps of heating the lignocellulosic biomass with steam to a preselected temperature, at a preselected pressure and for a preselected time to hydrolyze and solubilize hemicelluloses in the biomass; explosively decomposing the biomass into fibers; and extracting from the resulting reaction mixture a liquefied portion of the lignocellulosic biomass before or after explosive decomposition. The liquefied portion is extracted to remove compounds from the lignocellulosic biomass which are inhibitory to enzymatic cellulose hydrolysis and sugar fermentation to ethanol. For improved efficiency and economy, the inhibitory compounds are not completely removed. Furthermore, xylose has been found to be a good indicator compound for the general level of inhibitory compounds in the reaction mixture and the extraction step is therefore controlled on the basis of the xylose content in the reaction mixture. In particular, the extracting step is discontinued once a dry matter (dm) content of xylose, as monomer or oligomer, in the reaction mixture of 4% to 8% (w/w dm) is achieved. This most economically balances the practical need for inhibitory compound removal with the economical need to control and preferably minimize the costs of the overall ethanol production process.