To obtain data necessary for evaluation of hydrogen fluoride (HF) saccharification processes, a gravimetric techinque for measuring the quantity of HF vapor absorbed by lignocellulose was developed and used to measure absorption and desorption isotherms for dry bigtooth aspen wood (Populus grandidentata) for temperatures from 30 to 80 °C and HF partial pressures from 0 to 1 atm. Hysteresis was observed in the absorption-desorption loop, apparently due to cellulose decrystallization and saccharification. By use of the sugar yield data obtained elsewhere, the minimum HF requirement to achieve the maximum sugar yield was determined to be between 0.32 and 0.60 g of HF/g of wood. The heat of vaporization of HF from wood was calculated from the isotherm data using the Clausius-Clapeyron equation. Kinetic data for absorption and desorption were shown to be influenced by heat-transfer limitations at the conditions studied.