A method is described for forming a solution of cellulose in a tertiary amine oxide (such as N-methylmorpholine N-oxide) and a non-solvent for cellulose (such as water). The method involves the preparation of a mixture containing excess non-solvent, and removing the excess by heating the mixture to a temperature in excess of the boiling point of the non-solvent at a predetermined sub-atmospheric pressure, and feeding the heated mixture into the introduction zone of a mixing chamber at the predetermined sub-atmospheric pressure, so as to cause flash evaporation of some of the excess non-solvent. The mixture in the mixing chamber is then agitated by a rotor and transported through the mixing chamber whilst further evaporation of the non-solvent occurs. The cellulose solution thus formed is extracted from the mixing chamber, and the evaporated non-solvent withdrawn in cocurrent mode from the same end of the mixing chamber. Apparatus for carrying out the method is also described.