Microcrystalline cellulose is produced by subjecting to a high shear treatment, at elevated temperature and pressure, a reaction mixture of a cellulose material, an active oxygen compound and water, for a time effective to depolymerize the cellulose material. The mixture may be further depolymerized after the high shear treatment by holding it without cooling. A suitable active oxygen compound is hydrogen peroxide. An extruder is a typical high shear device.