Membrane fouling in electrodialysis (ED) can be minimized by using optimized pulsed power instead of direct current (DC) power. For the determination of an optimum frequency, effects of the square wave power with various frequencies on fouling mitigation were studied in the presence of humate as a foulant in a NaCl solution. In desalting of a NaCl solution with humate an optimum frequency of the square wave power was found to be 30 Hz, determined by the membrane fouling index for Neosepta AMX and CMX membranes. AMX membrane has a lower optimum frequency than that of the relatively loose AM-1, which was near 100 Hz in a previous study. Molecular weight distribution of humate transported to the concentrate compartment through AMX and AM-1 membranes indicated the structural difference between the two membranes. Through cell tests and desalting ED, a higher performance was achieved with the optimized square wave power than the DC power in teems of conductivity and cell resistance. (C) 2002 Elsevier Science B.V. All rights reserved.