In this paper, the influence of ultrasonic waves on capillary and viscous displacement of oil in porous media was investigated. Capillary (spontaneous) imbibition experiments were conducted using various fluid pairs such as air-water, mineral oil-brine, mineral oil-surfactant solution, kerosene-surfactant solution, and mineral oil-polymer solution. Berea sandstone and Indiana limestone cores were used as the matrix. Oil saturated cores were immersed into the aqueous phase and subjected to high intensity ultrasound from a specially designed ultrasonic chamber. The resulting recovery was recorded against time, and compared to a control experiment without ultrasound. A substantial increase in, ultimate recovery was observed for most of the fluid pairs, with some improvements in recovery rate. To further investigate whether ultrasound induces a perturbation at the liquid-liquid interface of immiscible fluids, a series of! Hele-Shaw type experiments were run, The resulting fingering pattern was strongly dependent on the interfacial tension of the fluid pair, and the injection rate. Ultrasound stabilized the fluid-fluid front of hi h interfacial tension fluid pairs, but generated larger instabilities when interfacial tension was low.