The present invention provides novel filled silicone compositions comprising a nano-particulate metal oxide filler selected from nano-particulate monoclinic alpha-phase bismuth oxide, nano-particulate erbium oxide, and mixtures thereof. The new composite compositions exhibit a combination of outstanding performance characteristics in both the cured and uncured states. Thus, the uncured formulations are typically free flowing liquids which are self-leveling, which de-gas readily under vacuum, are readily colorable through the addition of pigments, and which cure at temperatures of about 60° C. The cured compositions possess outstanding acoustic characteristics which make them ideally suited for use in applications such as acoustic lenses for medical imaging and therapy. For example, the acoustic impedance of compositions provided by the present invention is closely matched to the acoustic impedance of human tissue. This may result in an enhancement of ultrasound transmission efficiency between the lens of an ultrasound probe comprising such a composition and the patient.