Equilibrium moisture sorptions of two dried aqueous herbal extracts and their mixtures with dextrans of various molecular weights were investigated as a function of relative humidity at ambient temperature, and the data were analyzed by both the Guggenheim–Anderson–deBoer (GAB) and Brunauer–Emmett–Teller (BET) equations. Glass transition temperatures (Tg) of the samples were measured by differential scanning calorimetry, and their dependence on the moisture contents of the extracts was analyzed by the linear, Fox and expanded Gordon-Taylor mathematical models. All dextran–extract mixtures exhibited single Tg values, indicating that they existed as single homogeneous phases. The BET equation was found adequate for description of the moisture sorption isotherms for all samples. The dextrans appeared to reduce the hygroscopicity of the herbal extracts solely by a dilution effect. The observed increase in Tg and accompanying decrease in tackiness of the herbal extracts in the presence of dextrans may be explained by the ability of dextrans to restrict the molecular mobility of simple sugars and to counteract the plasticizing effect of water in the extracts. The expanded Gordon-Taylor equation has proved useful in predicting the Tg of hygroscopic amorphous herbal mixtures.