A method for the determination of the mass-transfer efficiency of pervaporation processes through semipermeable membranes is reported. Aliquots of ammonium ion solution were injected sequentially into two parallel manifolds with and without a pervaporation unit, both using the same reaction zone and detector. A channel which transports the Nessler's reagent was merged with the channels from both manifolds and the area of the peak obtained on passage of the reaction product through the photometric detector was used as the analytical signal. The variables influencing the efficiency of the pervaporation process (i.e., flow-rate, temperature, type of membrane, halting of the flow, etc.) were exhaustively checked in order to establish the working conditions for optimal development of the separation process.2982159165