Moisture content plays an important role both in the biochemical and thermochemical conversions of various feedstocks into energy. Knowledge of the relationship between relative humidity of air and moisture content of the feedstock is essential for drying and storage. The aim of this work was to determine the sorption isotherms of switchgrass and prairie cord grass and to compare the experimental data with isotherm models found in the literature. The equilibrium moisture content (EMC) of switchgrass and prairie cord grass was determined using the static gravimetric method at equilibrium relative humidities (ERH) and temperatures ranging from 12 to 89% and 20 degrees C to 40 degrees C. Depending upon the ERH values, the EMC values ranged from 9.2 to 20.8%, and 7.0 to 21.0% db for switchgrass and prairie cord grass, respectively and they followed typical the type II isotherm found in food materials. Non-linear regression was used to fit five commonly used three-parameter isotherm models to the experimental data: modified Oswin model, modified Halsey model, modified Chung-Pfost model, modified Henderson model, and the modified Guggenheim-Anderson-de Boer (GAB) model. Modified Halsey emerged as the best model followed by modified Oswin for both feedstocks with high F-statistic and R-2 values with low E-m and E-s and fairly random scattered residual plots. Because of the range of the environmental temperature and RH values, these models can be used to predict the equilibrium moisture content of these feedstocks starting from harvesting, drying, and preprocessing, through transportation, storage, and conversion/processing. (C) 2012 Elsevier Ltd. All rights reserved.