A comparative study on the microstructure of four ternary Al-Sn-Cu immiscible alloys, guided by the recent thermodynamic assessment of the system, was carried out with specific focus on the soft Sn particulate distribution in hard Al-rich matrix in the presence and absence of ultrasonic irradiation during solidification. The results clearly demonstrate high effectiveness of ultrasonication in promoting significantly refined and homogeneously dispersed microstructure, probably aided by enhanced nucleation and droplet fragmentation under cavitation. While conventional solidification produced highly segregated Sn phase at the centre and bottom of Sn-rich alloy ingots, ultrasonic treatment produced effective dispersion irrespective of the alloy constitution in the region around the radiator. At distance >20 mm from the radiator, presumably outside the region of active cavitation, any bulk fluid flow from acoustic streaming appears to be less efficient in preventing segregation of liquids even though still more effective than quiescent solidification conditions. (C) 2011 Elsevier B.V. All rights reserved.