Vibro-acoustography is a method that uses ultrasound radiation force at low frequency as an excitation mechanism to create images of the acoustic response of an object. At certain frequencies the magnitude image of an object consisting of materials of different densities may not provide sufficient contrast to distinguish each of the materials from one another so the phase of vibration is assessed as another modality for contrast. An experiment is described to address the efficacy of using the phase of vibration relative to the magnitude of vibration to assess mass density. Experimental data are compared with theoretical simulations. The authors discuss the usefulness of the phase images compared with the magnitude images. Possibilities for obtaining better contrast of materials of different density in a viscoelastic medium are explored