Stems of amaran thus are considered as prospective biofuel in the Czech Republic. The study presents results of water sorption tests of this biomass in the range of 10-30 degrees C and water activity (a(w)) ranging from 0.4 to 0.99. The experimental procedure used was a gravimetric dynamic method. Four sorption models (Chung-Pfost, Halsey, Henderson, Oswin) were evaluated. The modified Henderson's equation was the best model for moisture adsorption and desorption of amaranthus stems. Critical values of equilibrium moisture content, corresponding to the a(w) = 0.6, were 11.45% and 13.28% (wb) for water adsorption and desorption respectively, at the temperature of 20 degrees C. Heat of sorption (q(st)) was calculated using the chosen sorption model and Clausius-Clapeyron equation. An exponential function was found to fit the experimental q(st) values of amaranthus stems at moisture contents ranging from 7% to 40% (wb) for adsorption and desorption. (C) 2010 Elsevier Ltd. All rights reserved.