In the context of growing use of nanoparticles, it is important to be able to characterize all their physical properties in order to understand their behavior, to optimize them, and to control their quality. We showed that ultrasonic spectroscopy provides many of the desired properties. To do so, we used as an example nanocapsules made of a polymer shell encaspulating a liquid perfluorocarbon core and designed them for theranostic applications. Frequency-dependent measurements of both ultrasound velocity and attenuation were performed on nanocapsule suspensions. Then the desired properties were extracted by analyzing the experimental data using a recently developed model that relates the speed of sound and attenuation of a suspension to the geometrical and viscoelastic properties of the nanocapsules.