Phase images of palladium particles 5-15 nm in diameter were reconstructed from electron holograms acquired using the coherent beam of a field emission transmission electron microscope. The Pd particles were supported on amorphous silica microspheres, 0.2 mu m in diameter. A central contrast feature, suggestive of an internal void, was visible on most of the Pd particles. The phase profiles obtained from the holograms matched computed phase profiles confirming the existence of an internal void in these nano-particles. This is the first observation where internal morphology at the nano-scale has been unambiguously determined. While the observed voids are similar in scale to those occurring in recently reported hollow nanometre-sized particles, such as graphite polyhedra [9, 10], this represents the first report of such voids in metallic particles that are single crystals.