Separation of hydrazine from aqueous solutions with ethylcellulose membranes has been investigated by using the pervaporation technique. The effect of membrane thickness, concentration polarization, and feed concentration on flux and selectivity were evaluated. A separation mechanism is proposed based on the measurements of sorption, and diffusion coefficients, and estimations of Flory-Huggins interaction parameters and Hansen's solubility parameter. States of water, hydrazine, and hydrazine hydrate are explained with DSC spectra. The specific interaction sites in ethylcellulose matrix where the solvent interacts extensively with the polymer have been identified by FTIR analysis.