An equation for multimolecular sorption is presented. It is based on the BET model with the assumption that the adsorbing surface can be divided in two regions: A region that supports only a limited number of adsorbed layers due to geometric restrictions, and another region that can adsorb an unlimited number of layers. The proposed equation is applicable to type 2 and 3 isotherms (BDDT classification), and was successfully used to fit water sorption equilibrium data of some foodstuff from literature for the water activity range 0.05-0.90, and a temperature range from 10 to 70 degrees C. (C) 2013 Elsevier Ltd. All rights reserved.