Perfluorooctylbromide (PFOB: C8F17Br) has been shown to be an effective ultrasound contrast agent when incorporated in tissues. The authors have recently demonstrated that PFOB also enhances tissues and Doppler signals during its capillary phase. To help elucidate the physical basis of their observations, the authors compared physical and acoustical properties of standard PFOB-lecithin emulsion with identical lecithin emulsion minus PFOB (vehicle) for concentrations of PFOB from 0% to 50% weight per volume. Propagation velocity, attenuation, and elastic modulus were measured using a 5-MHz source, 0.6-mm hydrophone, and a waveform digitizer, whereas viscosity, density, and particle size were measured directly. Vehicle showed no significant change with increasing concentration in any of the parameters measured. Perfluoroctylbromide exhibited significant linear increase in viscosity and mass density. Increasing concentrations of PFOB emulsion produced a dramatic linear decrease in velocity (r = .99) and a linear increase in attenuation coefficient, (r = .99). Derived values for bulk modulus (reciprocal of compressibility) also showed significant linear decrease (r = .98) with concentration of PFOB. For typical human in vivo blood concentrations of 3.5% PFOB emulsion, the following values were obtained: density, 1.02 g/mL; viscosity, 10.25 cP; velocity, 1450 m/second; attenuation, 2.8 dB/cm; and bulk modulus, 2.18 GPa. Significant differences were found between PFOB and vehicle at even low concentrations, although particle number and size distributions were the same for each, indicating that the presence of PFOB strongly influences acoustic properties. This enhancement was found to be linear over a wide range of concentrations, which supports the possibility of quantitative analysis.