The use of high-intensity ultrasound to enhance the reactivity of metal surfaces has become a routine synthetic technique. In spite of this, the origin of rate enhancements in both stoichiometric and catalytic reactions remains unexplored. To this end, we have examined the effects of ultrasound on metal powders in terms of their particle and surface morphology, their atomic composition, and their reactivity. Specifically, we have discovered that ultrasonic irradiation of Ni and Cu powders leads to dramatic changes in morphology: individual surfaces are smoothed and particles are consolidated into extended aggregates. Surface composition was probed by Auger electron spectroscopy depth profiles, which revealed that ultrasonic irradiation removed the surface oxide coating, but also produced (especially for Cu) a deposition of surface carbon. The effects on reactivity can be substantial: ultrasonic irradiation enhances the activity of Ni powder as a hydrogenation catalyst by > 105 and significantly increases the reactivity of Cu powder as a stoichiometric reagent. We beleive that these effects are due to interparticle collisons driven by the turbulent flow created by the ultrasonic field. It is likely that the origin of the enhanced chemical reactivity comes from the removal of the surface oxide passivating layer which these collisions induce.