Sorption isotherms and glass transition temperature (T(g)) of powdered acai juice were evaluated in this work. Powders were produced by spray drying using different materials as carrier agents: maltodextrin IODE, maltodextrin 20DE, gum Arabic and tapioca starch. The sorption isotherms were determined by the gravimetric method, while the T(g) of powders conditioned at various water activities were determined by differential scanning calorimetry. As results, experimental data of water adsorption were well fitted to both BET and GAB models. Powders produced with maltodextrin 20DE and gum Arabic showed the highest water adsorption, followed by those produced with maltodextrin 10DE and with tapioca starch, respectively. With respect to the glass transition temperature, Gordon-Taylor model was able to predict the strong plasticizing effect of water on this property. Both a(w) and T(g) were used to determine the critical conditions for food storage, at which powders are not susceptible to deteriorative changes such as collapse, stickiness and caking. (C) 2009 Elsevier Ltd. All rights reserved.