A process is described for pretreating lignocellulosic biomass. The process comprises swelling the lignocellulosic biomass with an aqueous liquid. The pretreated lignocellulosic biomass may be used as a feedstock for the enzymatic conversion to ethanol, or in a thermal conversion. process to produce bio-oil. The pretreatment results in a greater yield and, in the case of a thermal conversion process, a better quality of the bio-oil. The pretreatment process may be used to adjust the composition and amount of inorganic material present in the lignocellulosic biomass material.