The phase velocities in suspensions of Albunex, an ultrasound contrast agent, are determined using the technique of broadband phase spectroscopy. In these suspensions, phase velocities were measured as a function of Albunex concentration in Isoton II (buffered saline). The dispersion over the measurement bandwidth (1-15 MHz) grew with increasing Albunex concentration, exhibiting a 221-m/s change (17% increase) in phase velocity between 1.0 and 3.8 MHz at the highest concentration reported (32 microL Albunex/8 mL Isoton). The dispersion behavior of the Albunex suspensions is described using a model of encapsulated gas bubbles. The influence of the dispersion in Albunex on broadband pulses is discussed, as well as the potential impact this dispersion may have on the formation of ultrasonic images of the heart.