NOVELTY - Processing cellulose-containing biomass comprises subjecting a treatment mixture comprising the cellulose-containing biomass, water and methanesulfonic acid to a certain temperature and pressure.    USE - The method is used for processing cellulose-containing biomass. The methanesulfonic acid is used for processing cellulose-containing biomass, especially for the pretreatment of cellulose-containing biomass prior to saccharification, where the methanesulfonic acid is used (i) in the presence of the compounds selected from compounds of formula (I) and surfactants which are not compounds of formula (I); or (ii) in the absence of any compound selected from the compounds of formula (I) and any surfactant which is not a compound of formula (I) (all claimed).    ADVANTAGE - The method for processing cellulose-containing biomass above allows to reduce or even to avoid the use of aggressive chemicals like sulfuric acid without compromising the yield of glucose obtainable by saccharification of the treated cellulose-containing biomass. The method results in an increased yield of glucose obtainable by saccharification of the treated cellulose-containing biomass. The method is done in the absence of surfactants and other additives.    DETAILED DESCRIPTION - Processing cellulose-containing biomass comprises subjecting a treatment mixture comprising the cellulose-containing biomass, water and methanesulfonic acid to 100-200 degrees C at a pressure of 1-20 bars (100-2000 kPa), where the pressure is selected so that a part of the water is in the liquid state to generate a treated cellulose-containing biomass. The (i) treatment mixture further comprises the compounds selected from compounds of formula (I) and surfactants which are not compounds of formula (I); or (ii) treatment mixture does not comprise any compound selected from compounds of formula (I) and does not comprise any surfactant which is not a compound of formula (I). The R1 and R2 are selected from hydrogen, unsubstituted and substituted alkyl with 1 to 22 carbon atoms (C1 to C22-alkyl) and unsubstituted and substituted aryl, where in the substituted C1 to C22-alkyl and the substituted aryl each substituent is independently selected from -OSO3H, -SO3H, -COOH and -OPO3H2 and their salts. Each Rx in the x groups shown in structure (II) is a further Rx selected from hydrogen and alkyl with 1 to 20 carbon atoms (C1 to C20-alkyl) and x is 1-2400. An INDEPENDENT CLAIM is treated cellulose-containing biomass obtainable according to the method above.