Microwave-based methods are increasingly being used to synthesize metal-metal oxide nano-heterostructures. However, the detailed mechanism of formation of such hybrids has not been addressed. In this work we illustrate the thermodynamic and kinetic aspects of the reduction of metal salts by ethylene glycol under microwave dielectric heating. Based on this investigation, we identify the temperature above which the reduction of the metal salt is thermodynamically favorable and temperatures above which the rates of homogeneous nucleation of the metal and the heterogeneous nucleation of the metal on the oxide supports are favored. The effect of interfacial energy of the metal/solvent in the case of homogeneous nucleation and the metal/support in the case of heterogeneous nucleation on the kinetics of nucleation is illustrated. Thus we define different conditions which favor the heterogeneous nucleation of the metal on the supports over homogeneous nucleation in the solvent medium based on the dielectric loss parameters of the solvent and the support and the metal/solvent and metal/support interfacial energies. Contrary to current understanding, we show that metal particles can be selectively formed on the substrate even under situations where the temperature of the substrate is lower than that of the surrounding medium. Theoretical predictions are supported by the experimental observations of Pt reduction under conditions of homogeneous and heterogeneous nucleation. Exploiting the selective heterogeneous nucleation of the metal on substrates with high loss tangent, under microwave heating conditions, nanoparticles of Pt and Au are synthesized on CeO2, TiO2 and ZnO supports which are extensively used as heterogeneous catalyst. Our method thus opens up possibilities for rational synthesis of high activity supported catalysts using a fast microwave-based reduction method.