Self-diffusion measurements of dimethyl sulfoxide and dimethylformamide in solutions and gels of cellulose acetates have been carried out by pulsed field gradient NMR for different concentrations and degrees of substitution of the polymer. The solvent’s diffusion coefficients, D, are found to depend on the time of measurement, indicating the system to be heterogeneous within a space scale of a few microns. D time dependence is more explicit for higher concentrations of the polymer with lower substitution degree.