Temperature programmed desorption (TPD), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM) have been used to characterize molybdenum carbide nanoparticles prepared on a Au(111) substrate. The MoCx nanoparticles were formed by Mo metal deposition onto a reactive multilayer of ethylene, which was physisorbed on a Au(111) substrate at low temperatures (<100 K). The resulting clusters have an average diameter of similar to1.5 nm and aggregate in the fee troughs located on either side of the elbows of the reconstructed Au(111) surface. Core level XPS shows that the electronic environment of the Mo and C atoms in the nanoparticles is similar to that found in Mo2C-(0001) single crystals and carburized Mo metal surfaces. Peak intensities in XPS and AES spectra were used to estimate an average Mo/C atomic ratio of 1.2 +/-0.3 for nanoparticles annealed above 600 K.