A miniaturized ultrasonic probe has been constructed to measure the local volume fraction of immiscible phases in liquid-liquid contactors. The new probe has a 9 mm diameter and varying length and is based on sound pulse-echo measurements. It consists of a sound reflector and an ultrasonic transducer which operates as a transmitter-receiver. The probe has been successfully used to measure the dispersed phase transient volume fraction (holdup) in stirred tanks. Measurements between 0.08 and 0.8 were obtained at the top, bottom, and impeller discharge regions of the tanks. It has been found that local variations in the holdup exist, due to density differences of the two phases, which can be quantified by the new pulse-echo ultrasonic probe. Furthermore, measurements were obtained during phase inversion at which the continuous phase becomes dispersed and the dispersed phase continuous. Scattering and discontinuity is observed in the measurements in the proximity of phase inversion, which may be useful to determine which phase is dispersed and which is continuous.