The parameters of the 'G.A.B.' equation are discussed from mathematical and physical point of view. Clear limits of those parameters are determined. The point of inflexion of the 'G.A.B.' isotherm was also analysed with the conclusion that the 'G.A.B.' isotherm behaves too rigidly to be able to describe complicated sorption cases. The different possibilities of some other isotherms for such a purpose were displayed. (C) 2004 Elsevier Ltd. All rights reserved.