A new equation is developed in this paper which is further used to divide sorption isotherms into classes which are connected to surface water effects and/or solution water effects and their combinations. It is shown that sorption isotherms can be successfully classified using the new equation into three basic classes I, II (most frequent) and III which roughly correspond to the original Brunauer's classes. The equation is also used for further division of class II into three subclasses. Criteria for these classifications are presented. (C) 2008 Elsevier Ltd. All rights reserved.