The ability of ultrafiltration membranes to effect a separation of the volatile essential oil fraction of guayule resin from other volatile constituents is experimentally demonstrated. For a methanol extraction of guayule resin, 200 and 500 molecular weight cut-off cellulose triacetate ultrafiltration membranes produced a permeate rich in volatile essential oils, evidenced by gas chromatographic analyses, The filtration process was not influenced by concentration polarization up to a transmembrane pressure gradient of about 400 kPa, or permeate flux rates of 13 x 10(-7) m/s and 18 x 10(-7) m/s for the 200 and the 500 molecular weight cut-off membranes respectively.