The water adsorption isotherm and glass transition temperatures (Tg) of chicken protein hydrolysate powder, with and without maltodextrin or gum Arabic, were studied in order to investigate their stability. The hydrolysate powder, pure and formulated with 10%, 20% and 30% (w/w) of additive, was obtained by spray drying. The sorption isotherm was determined by the gravimetric method. A differential scanning calorimeter was used to determine the Tg of samples equilibrated with several water activities. As results, the BET model fitted the data for the sorption isotherm of the protein hydrolysate well. A strong plasticizing effect of water on the Tg was found, with a great reduction in this value with increase in water activity. The data for Tg versus solids content gave a satisfactory correlation with the Gordon-Taylor model. The addition of carrier agents increased the Tg of the hydrolysate, decreasing its hygroscopicity and, consequently, increasing its storage stability.