We report measurements of the sound velocity diffusivity and viscosity of aqueous solutions of alpha-alpha-trehalose at temperatures between 20 and 85 degrees C for volume fractions from 0.041 to 0.705. The ultrasonic data show that the mixing process is not ideal and that the trehalose-water interaction strength, because of the saturation of H-bond sites, decreases with increasing concentration. Photon correlation spectroscopy reveals marked changes in the effective diffusion coefficient in the dilute region over a limited temperature range. These measurements, interpreted in conjunction with viscosity data on the basis of a model by Mooney and Padova, quantify the hydration properties of alpha-alpha-trehalose. The dependence on volume fraction and the sensitivity to temperature changes characterize the "fragility" of this disaccharide-water system.