An apparatus for pervaporation of liquid mixtures by flat polymer membranes is described. Two different methods for determination of pervaporation fluxes - the classic direct gravimetric method (whereby the pervaporate is condensed and then weighed) and the sorption method (where the pervaporate is adsorbed into the active carbon and the mass is determined from the change in length of the quartz spring balance on which the active carbon is suspended) are described and the results are compared with the system 1-butanol-toluene-low density polyethylene membrane at 25^oC. It was found that over shorter periods of time the sorption method gave more accurate results even for very small fluxes than with the direct method.