Ethylcellulose membrane has been selected on the basis of Hansen＇s solubility parameter and Flory-Huggins interaction parameter for the enrichment of hydrazine and monomethylhydrazine (MMH) liquid propellants by pervaporation. An extensive study of the overall mass transfer resistance experienced by the permeants has been conducted. The resistance values were quantitatively estimated by changing the membrane thickness and calculating the corresponding flux. Due to its lower sorption, and fewer interactions, the membrane showed least desorption resistance towards water and thus it is permselective with respect to water. Results of pervaporation selectivity obtained in separation of water-hydrazine and water-MMH mixtures at azeotropic compositions have been correlated. Higher sorption of MMH and hydrazine did not result in preferential separation inspite of lower membrane resistance. Experimental results clearly showed that desorption resistance and diffusivity were predominant over the respective solubilities. To confirm the reasons for these phenomena, FTIR and DSC spectra of membranes soaked in pure hydrazine, MMH, water and hydrazine hydrate were compared. (C) 2000 Elsevier Science Ltd. All rights reserved.