A simple, microwave-assisted, strategy for producing Au/Ag concentrated sols by glucose reduction in water was developed. Ag-Au bimetallic nanoparticles stabilized by polyvinylpyrrolidone (PVP) were characterized and their catalytic activity was studied in the reduction of 4-nitrophenol with NaBH4 as a probe reaction. The Ag-Au nanoparticles were prepared by first optimizing the synthesis of Au colloid and then carrying out the deposition of a silver shell. Microwave heating has been shown to provide more homogeneous particle nucleation and shorter synthesis time than traditional heating. Prepared Au, Ag and Au/Ag nanocrystals function as effective catalysts for the reduction of p-nitrophenol in the presence of NaBH4, otherwise unfeasible if only the strong reducing agent NaBH4 is employed.