TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The distal end of the counter current reaction vessels further comprises an outlet port. The counter current reaction vessel is a screw extruder or conveyor. The screw extruder or conveyor is a multi-screw extruder or conveyor, or twin-screw extruder or conveyor. The twin-screw extruder is a co-rotating twin screw extruder.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: In the method (M1): the initial temperature of the hydrolysis fluid is at least 140 degrees C; the pressure within the reaction vessel exceeds the maximum saturation vapor pressure of the hydrolysis fluid and fluidized composition; and water is introduced at or near the distal end of the reaction vessel for rinsing of any residual solid prior to its discharge from the reaction vessel. In the method (M2), the mean residence time of the solid biomass feedstock within the reaction vessel is less than or equal to 60 minutes. Preferred Components: The polycarbohydrate comprises cellulose, hemicellulose and/or lignin. The biomass feedstock further comprises hemicellulose and/or lignin. The fluidized composition further comprises: furfural, hydroxymethylfurfural, levulinic acid, formic acid and/or acetic acid; and an acetonated carbohydrate. The organic solvent comprises an alcohol and/or a ketone. The alcohol comprises methanol, ethanol, propanol and/or butanol. The ketone comprises acetone. The initial pH of the hydrolysis fluid is 0-6.5, preferably less than or equal to 5.5. The acid comprises a carboxylic acid, phosphoric acid, phosphorous acid, nitric acid, sulfuric acid, sulfurous acid, sulfamic acid, sulfonic acid, hydrochloric acid and/or carbonic acid. The carboxylic acid is a monocarboxylic acid and/or a dicarboxylic acid, preferably maleic acid, acetic acid, propionic acid, formic acid and/or levulinic acid. The fluidized composition comprises at least 40 wt.%, preferably at least 90 wt.% of the solid biomass feedstock. EXAMPLE - No suitable example given.