A new definition of the concentration-dependent "shielding factor" has been proposed, along with its particular value at a concentration assumed to be equal to critical micelle concentration (cmc). It has been shown that herein defined shielding factor enables one to calculate the values of monomer (a(x)) and counterion activities (a(y)) which are in better agreement with the measured ones than those calculated using Burchfield and Wooley's constant value of the shielding factor. In addition, it was shown that by using particular value of shielding factor at cmc, one is able to calculate the micellar charge radius which is dependent on aggregation number, counterion parameter and cmc, all of them having important implications for thermodynamic treatment of the micellization process. (c) 2005 Elsevier Inc. All rights reserved.