The topic dealt with in this paper concerns an experimental study, performed by means of ultrasonic techniques, on aqueous solutions of alpha,alpha-trehalose, an effective bioprotector against dehydration. The ultrasonic velocity and density data show that, in these aqueous solutions, the mixing process is not ideal. The behavior of the excess compressibility as a function of concentration can be interpreted by supposing that alpha,alpha-trehalose molecules form intramolecular hydrogen bonds by folding over at high concentration. When water molecules are added, they spread out again, thereby releasing hydrogen bond sites for water interactions and increasing the water-sugar interaction strength. Furthermore, the temperature dependence of the hydration number has been evaluated, and its value at room temperature is very close to that given in the literature.