Specific interaction sites of ethylcellulose (EC) with water and monomethyl hydrazine (MMH) and its hydrate have been studied by Fourier transform infrared spectra. Measurements of diffusion and sorption coefficients of MMH and water in EC were made from the reduced sorption curves to determine the overall selectivity of the membrane. Flow patterns across the membrane were examined under an optical microscope (OM) equipped with a differential interference contrast (DIC) facility. Correlation of the front velocity and the diffusivity has been used to calculate and compare these values of diffusion coefficient with that obtained from reduced sorption curve. Desorption of MMH hydrate from EC has been attributed to polymer relaxation phenomena. The physical aging process of the membrane has been monitored by FTIR analysis and mechanical strength evaluation. These studies showed that the EC membrane can be used for prolonged periods in MMH and, as such, is suitable for the separation of MMH-water solutions.