TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Process: The flash cooling is performed in only one step or is performed in at least two steps. The entire flash cooling, performed in at least one step, is performed at 40-280 degrees C, preferably at 100-230 degrees C, especially at 100- 210 degrees C. A first flash cooling step is performed at 190-220 degrees C and a second flash cooling step is performed at 100-190 degrees C. The flash cooling is performed in at least one flash tank, which is preceded by a pressure reduction device. The aqueous monomer and/or oligomer sugar mixture subjected to the flash cooling comprises water soluble hemicelluloses, solid cellulose and lignin, and where the process results in a hydrolyzed delignified solution of sugar monomer and/or oligomers; or the aqueous monomer and/or oligomer sugar mixture subjected to the flash cooling comprises water soluble cellulose oligomers and solid lignin, and where the process results in a hydrolyzed delignified solution of sugar monomer and/or oligomers; or the aqueous monomer and/or oligomer sugar mixture subjected to the flash cooling comprises water soluble sugar components originating from hemicelluloses and cellulose, and solid lignin, and where the process results in a hydrolyzed delignified solution of sugar monomer and/or oligomers; or the aqueous monomer and/or oligomer sugar mixture subjected to the flash cooling comprises water soluble sugar components originating from hemicelluloses and/or cellulose. Generated flash vapor is used to heat other process operations. A pressure control device is arranged in the process just upstream of a flash vessel, and is either i) control/throttling valve, ii) small inner diameter pipe, iii) an orifice plate, and/or iv) reverse acting pump or v) a combination of any of (i)-(iv). The process also involves distillation, adsorption, absorption, filtration and/or separation and recovery of byproducts; and adding an additive in the flash cooling step, where the additive is a base and/or a defoamer. The step of flash cooling is preceded by the hydrothermal, dilute acid hydrolysis performed as a thermal treatment in either one step or several steps. The step of flash cooling is preceded by a first thermal treatment step in which the biomass feedstock is subjected to treatment with hot compressed liquid water (HCW) at subcritical conditions and/or steam during a residence time T1, and a second hydrolysis step in which the lignocellulosic biomass feedstock is further treated in at least hot compressed liquid water (HCW) at subcritical conditions during a residence time T2 for the depolymerization of carbohydrates to produce an aqueous monomer and/or oligomer sugar mixture. At least one of the steps of thermal treatment involves a pH decrease, where the pH value during the thermal treatment is at most 4, preferably 1 -4, especially 1.2-3.3. The hydrothermal, dilute acid hydrolysis comprises or is preceded by the addition of inorganic and/or organic acids. The hydrothermal, dilute acid hydrolysis is performed in one step at a temperature of at least 200 degrees C or in at least two steps where a first thermal treatment step is performed at a temperature of at least 170 degrees C and a second treatment step at a temperature of at least 200 degrees C; or the hydrothermal, dilute acid hydrolysis is performed in one step at 220-280 degrees C or in at least two steps where a second or later thermal treatment step is performed at 220-280 degrees C. The flash cooling is combined with cold or tempered water injection, with or without sugar monomers and oligomers, in one or several steps. The flash cooling is performed in a first flash unit at 190-220 degrees C and where residence time is no longer than 10 minutes in the first flash unit. The total solid content of the biomass feedstock during the thermal treatment is 5-90%, preferably 10-50%. The hydrothermal, dilute acid hydrolysis is performed in at least two steps and where dissolved water soluble compounds are separated from a solid residue after the first step to prevent continued detrimental degradation. The hydrothermal, dilute acid hydrolysis is performed in at least two steps and where a solid residue after the first step is rinsed from water soluble compounds by washing with water, followed by additional liquid-water separation. The step of flash cooling is preceded by the hydrothermal, dilute acid hydrolysis performed as a thermal treatment in either one step or several steps and where a solid residue is rinsed from water soluble compounds, followed by additional liquid-water separation, in a repeated fashion. The step of flash cooling is preceded by the hydrothermal, dilute acid hydrolysis performed as a thermal treatment in either one step or several steps and where a solid residue is rinsed from water soluble compounds by adding acidified water in a last washing step. EXAMPLE - No suitable example given.