Freeze-drying is a common process for the long-term preservation of biochemicals derived from aqueous solutions. One of the important factors involved in a successful freeze-drying procedure is the secondary drying step. The effect of temperature (254-293 K) and pressure (5-44Pa) on water mass transport during the secondary drying stage has been examined for concentrated, aqueous solutions of hydroxyethyl starch, a stabilizing agent common in the freeze-drying of biological material. Desorption equilibria were reached after evacuation for 5-8 d. The equilibrium water content W showed a significant variation with water activity a(w). Based on a Freundlich sorption model, the equilibrium water content may be described by the equation W = 0.578 a(w)0.515 Water diffusion coefficients were derived from the equilibration behaviour according to Fick’s diffusion law and were found to vary over the range 1.0-2.8 x 10(-11) m(2)s(-1). The mass transport properties and their dependence on temperature, pressure and composition are discussed in detail. The importance of these results for a comprehensive understanding of the freeze-drying process, efficient process design and final product quality are emphasized.