The amount of water-insoluble fibers in a water-soluble cellulose derivative is reduced in a process comprising the steps of a) providing a water-soluble cellulose derivative having a residual amount of at least 20 ppm by weight of water-insoluble fibers in a 2 weight percent aqueous solution of the water-soluble cellulose derivative; b) mixing the water-soluble cellulose derivative of step a) with a liquid in a compounder to provide a moist water-soluble cellulose derivative having a temperature of at least 50 C and a moisture content of from 35 to 90 percent, based on the total weight of the moist cellulose derivative; and c) drying-grinding the mixture of step b) in a gas-swept impact mill to obtain a dried and ground cellulose derivative.