A method for treating a mixture in a single-shaft or multi-shaft mixer (M), especially a kneader-mixer, especially for preparing a spinning solution. A solvent or solvent mixture is added to the product over the length of a product chamber in order to reduce a viscosity of the solvent or of the mixture and to increase an evaporative capacity. The viscosity of the solution or of the mixture is determined and/or modified in predetermined locations in the product chamber.