The reduction behavior of a Pd-Cu/gamma-Al2O3 catalyst precursor (containing 2% Pd and 1% Cu) is studied by atomic scale Z-contrast imaging, electron energy-loss spectroscopy (EELS), and X-ray energy dispersive spectroscopy (EDS) techniques available in a scanning transmission electron microscope (STEM). We found that the alloying behavior of the bimetallic nanoparticles strongly depends on the reduction temperature of the catalyst precursor materials. When the precursor is reduced at 523 or 773 K, individual metallic nanoparticles are formed with a composition varying from pure metallic Pd to Pd-Cu bimetallic alloys. Detailed spectroscopic analyses of the individual nanoparticles show that Pd is preferentially segregated onto the surfaces of the bimetallic Pd-Cu nanoparticles. At higher reduction temperatures, e.g., at 1073 K, however, all the nanoparticles are found to be bimetallic Pd-Cu alloys with either Pd- or Cu-rich surfaces.