TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Process: The extraction step (a) is: performed at a temperature of 30-80 degrees C; and preceded by a pre-extraction step in which the biomass is extracted with water at a temperature below 100 degrees C, preferably 20-60 degrees C. The treatment step (b) is: performed at a temperature of 125-250 degrees C, preferably 120-170 degrees C, and at a pH of 1-5; and an organosolv treatment, where the second organic solvent is preferably lower alcohols or ethers. The process further comprises (c) subjecting a cellulose-enriched product stream resulting from step (b) to enzymatic hydrolysis. The step (c) is preformed in the presence of an aqueous extract obtained in the pre-extraction step. Preferred Components: The first organic solvent is: a water-miscible solvent comprising lower alcohols, ethers or ketones; and the same as the second organic solvent, preferably both comprise ethanol or acetone. The extracting liquid comprises 0-30 vol.% of water. The treatment liquid comprises 25-80 wt.% of the second organic solvent, based on total weight of the treatment liquid, 20-75 wt.% water, based on total weight of the treatment liquid, and at most 500 g of an acid, per kg dry weight of the biomass. EXAMPLE - First batch (batch 1) of wheat straw was pre-extracted using 10 l water per kg biomass, and a second batch (batch 2) of wheat straw was first pre-extracted using the same conditions as for the first batch, and subsequently extracted with 10 l ethanol (containing 4 wt.% water) per kg biomass. Both pre-extraction and extraction were performed using a Soxhlet extractor. Both batches 1 and 2 of wheat straw were subjected to organosolv. Organosolv conditions employed a temperature of 140 degrees C for 120 minutes, 60/40 wt/wt.% ethanol/water (as solvent), liquid/solid ratio of 10 l/kg, and 50 mM sulfuric acid for the (pre-)extracted wheat straw. Thus after extraction, the batch 1 was fractionated to contain 91.7 wt.% biomass recovery after extraction, 47.6 wt.% pulp based on dry weight of the fresh biomass before (pre-)extraction, 82.4% xylan hydrolysis, based on amount of xylan present in the fresh biomass and 70.8 % lignin removal, based on lignin present in the fresh biomass, and the batch 1 was fractionated to contain 87.6 wt.% biomass recovery after extraction, 40.2 wt.% pulp based on dry weight of the fresh biomass before (pre-)extraction, 92.8% xylan hydrolysis, based on amount of xylan present in the fresh biomass and 87.5 % lignin removal, based on lignin present in the fresh biomass.