The present invention describes a process for a controlled conversion of a biomass feedstock, wherein the process comprises the steps of: loading the biomass feedstock to at least one reactor; liquefaction of the biomass feedstock into a monomer and/or oligomer sugar mixture in said reactor by treatment in hot compressed liquid water (HCW) at sub- and/or super-critical condition; and removal of the monomer and/or oligomer sugar mixture, being the product molecules, to avoid continued detrimental decomposition.