The study focused on the dual role of water as a cosolvent and an antisolvent in the [HSO 4 ]-based protic ionic liquid biomass fractionation process using N , N , N -dimethylbutylammonium hydrogen sulfate, [DMBA]­[HSO 4 ]. The effectiveness of biomass fractionation using [DMBA]­[HSO 4 ] mixed with different concentrations of water of conventional biorefinery feedstocks (Miscanthus and pine softwood) and nonconventional low-cost lignocellulosic biomass waste (treated timber and postconsumer waste wood) was investigated. The pulp composition, lignin extraction, and enzymatic hydrolysis of the cellulose pulp were analyzed after pretreatment at 170 °C for 30 min. We showed that it is possible to reduce the ionic liquid use in the process by increasing the water concentration as a cosolvent while still maintaining the effective biomass deconstruction forMiscanthus and postconsumer waste wood. However, softwood biomass showed higher resistance to fractionation at higher water concentrations in the pretreatment medium. We also investigated the impact of reducing the amount of water used as an antisolvent for lignin precipitation in terms of lignin yields and properties. The robust performance of the fractionation process at the optimized antisolvent use was demonstrated using the challenging feedstock pine softwood over six pretreatment cycles. Finally, we demonstrated the significance of evaluating water use for the energy requirements of the process, particularly in the ionic liquid regeneration step, achieving a 65% energy reduction.