NOVELTY - The process comprises: loading the biomass feedstock to a reactor; liquefying the biomass feedstock into a monomer and/or oligomer sugar mixture in the reactor by treatment in hot compressed liquid water (HCW) at sub- and/or super-critical condition; removing the monomer and/or oligomer sugar mixture; and removing non-solubilized material. The monomer and/or oligomer sugar mixture is present with product molecules to avoid continued detrimental decomposition. The liquefaction is performed at a temperature 280-350 degrees C during a time of 1.5-30 seconds.    USE - The process is useful for a controlled conversion of a biomass feedstock such as a lignocellulosic biomass feedstock and slurry (all claimed).    ADVANTAGE - The process is capable of effectively and economically controlling conversion of the biomass feedstock with improved productivity.    DETAILED DESCRIPTION - The process comprises: loading the biomass feedstock to a reactor; liquefying the biomass feedstock into a monomer and/or oligomer sugar mixture in the reactor by treatment in hot compressed liquid water (HCW) at sub- and/or super-critical condition; removing the monomer and/or oligomer sugar mixture; and removing non-solubilized material. The monomer and/or oligomer sugar mixture is present with product molecules to avoid continued detrimental decomposition. The liquefaction is performed at a temperature 280-350 degrees C during a time of 1.5-30 seconds, and is performed at a sub-critical condition implying the temperature of 374 degrees C. The step of loading the biomass feedstock is performed by: preloading biomass into a batch reactor; cyclic loading of biomass into a single batch reactor or into a series of batch reactors so that the reactors are refilled after complete biomass liquefaction; and continuous loading of biomass into a flow reactor. The HCW is injected to the batch and flow reactors: by a cycle or repeated cycles; and or series of batch reactors by a cycle or by repeated cycles. A solubilized material is: discharged from the batch reactor after a reaction time; and flowed downstream from the flow reactor to a non-reactive zone. A separation of a lignin component is performed by filtration/removal of non-solubilized material from the batch reactor or series of batch reactors. The flow reactors are a loading reactor and a reaction reactor. The slurry is continuously loaded to a flow reactor and is rapidly warmed to sub- or super-critical condition. The removed non-solubilized material is reprocessed. The lignin is separated at the step of removal of non-solubilized material. The process is performed free from any chemicals besides HCW and the biomass feedstock. A pH lowering additive is added to the process. Each injection of HCW implies applying a temperature pulse at a sub- or super-critical condition in the reactor to allow for liquefaction of the biomass feedstock. The pulse applies a pulse start temperature during a temperature increase time and allowing for liquefaction reaction to occur during the reaction time. The pulse start temperature is 300-350 degrees C, and the reaction time is set to 1.5-15 seconds. The process further comprises hydrolyzing and/or fermenting the feedstock.