A process for producing a nanocellulose material or a nanocellulose-containing product comprising nanocellulose particles having at least one length dimension of from 0.01nm to 1000nm, said process comprising: (a) providing a lignocellulosic biomass feedstock; (b) fractionating said feedstock in the presence of an acid, preferably an acid selected from the group consisting of sulfur dioxide, sulfurous acid, sulfur trioxide, sulfuric acid, lignosulfonic acid, and combinations thereof, and more preferably an acid of sulfur dioxide, a solvent for lignin, and water, to generate cellulose-rich solids and a liquid containing hemicellulose and lignin; (c) mechanically treating said cellulose-rich solids with a total mechanical energy of less than about 1000 kilowatt-hours per ton (1.102 kWh/kg) of said cellulose-rich solids to form cellulose fibrils and/or cellulose crystals, preferably wherein said cellulose-rich solids are treated with a total mechanical energy of less than about 500 kilowatt-hours per ton (0.551 kWh/kg) of said cellulose-rich solids, and more preferably with a total mechanical energy of from about 100 kilowatt-hours(0.110 kWh/kg) to about 400 kilowatt-hours per ton (0.441 kWh/kg) of said cellulose-rich solids, thereby generating a nanocellulose material having a crystallinity of at least 60%, preferably of at least 70%, 80% or 85%; and (d) (i) recovering said nanocellulose material; or (ii) incorporating at least a portion of said nanocellulose material into a nanocellulose-containing product, preferably wherein said nanocellulose-containing product consists essentially of said nanocellulose material. (a) providing a lignocellulosic biomass feedstock; (b) fractionating said feedstock in the presence of an acid, preferably an acid selected from the group consisting of sulfur dioxide, sulfurous acid, sulfur trioxide, sulfuric acid, lignosulfonic acid, and combinations thereof, and more preferably an acid of sulfur dioxide, a solvent for lignin, and water, to generate cellulose-rich solids and a liquid containing hemicellulose and lignin; (c) mechanically treating said cellulose-rich solids with a total mechanical energy of less than about 1000 kilowatt-hours per ton (1.102 kWh/kg) of said cellulose-rich solids to form cellulose fibrils and/or cellulose crystals, preferably wherein said cellulose-rich solids are treated with a total mechanical energy of less than about 500 kilowatt-hours per ton (0.551 kWh/kg) of said cellulose-rich solids, and more preferably with a total mechanical energy of from about 100 kilowatt-hours(0.110 kWh/kg) to about 400 kilowatt-hours per ton (0.441 kWh/kg) of said cellulose-rich solids, thereby generating a nanocellulose material having a crystallinity of at least 60%, preferably of at least 70%, 80% or 85%; and (d) (i) recovering said nanocellulose material; or (ii) incorporating at least a portion of said nanocellulose material into a nanocellulose-containing product, preferably wherein said nanocellulose-containing product consists essentially of said nanocellulose material. (i) recovering said nanocellulose material; or (ii) incorporating at least a portion of said nanocellulose material into a nanocellulose-containing product, preferably wherein said nanocellulose-containing product consists essentially of said nanocellulose material.