Semiconducting nanoparticles of indium and thallium oxides (In 2O3 and Tl2O3) have been successfully synthesized in high yield (> 95%) by microwave irradiation. The oxides are synthesized in a simple domestic microwave oven (DMO) by adding an aqueous ammonia solution to the aqueous solutions of indium chloride (InCl 3) and thallium chloride (TlCl3), respectively. The structures, morphologies, compositions and physical properties of the products have been characterized by powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), energy dispersive X-ray (EDX) analysis, transmission electron microscopy (TEM), selected area electron diffraction (SAED), etc. The XRD and SAED analysis indicate that the as-synthesized product, obtained after microwave (MW) heating of the indium chloride solution, is crystalline In(OH)3, but its calcined form is a crystalline body-centered cubic (bcc) phase of In2O3. On the other hand, in the case of thallium chloride, the as-synthesized material is a crystalline body-centered cubic (bcc) phase of Tl2O3. The particles are of uniform size, being about 16 nm and 24 nm for Tl 2O3 and In2O3, respectively. A possible mechanism for the formation of the In2O3 and Tl2O3 nanoparticles is discussed briefly.