The regeneration of a solid, crystallized cellulose solution in a N-methylmorpholine-N-oxide (NMMO)-water mixture was studied by measuring the diffusion coefficient of both the water uptake from the regenerating bath and the NMMO outflow to this bath. The diffusion coefficient of water going to the cellulose solution is about 10 times larger than the diffusion coefficient of NMMO leaving the solution. This difference expresses the strongly hygroscopic character of NMMO. None of these coefficients depends on cellulose molecular weight showing that no major rearrangement of cellulose chains occurs at the beginning of the regeneration. The diffusion coefficient of water is not influenced by the cellulose concentration, whereas the diffusion coefficient of NMMO decreases strongly when the cellulose concentration increases. Extrapolating the diffusion coefficient of NMMO versus cellulose concentration to zero shows that the maximal concentration of cellulose in NMMO-water is about 15%. Above this value, undissolved cellulose should be present. From the influence of the NMMO content in the water regenerating bath, it is possible to see that NMMO is removed from the solution if the bath has a NMMO content lower than 60%, to be compared with the 80% NMMO concentration in the solution.