Specific interaction sites of ethylcellulose with water, hydrazine, and the mixtures of hydrazine hydrate and water have been studied by FTIR spectra. Measurements of diffusion and sorption coefficients of these materials in ethylcellulose were made to determine the overall selectivity. Aging studies of the membrane including an accelerated aging test have also been conducted. The influence of aging on physico-chemical and mechanical properties were assessed by FTIR studies and mechanical tests. These studies showed that the ethylcellulose membrane can be used for prolonged periods in hydrazine hydrate and as such is suitable for the separation of hydrazine-water mixtures.