The diffusion and coalescence of Au-Cu alloy nanoparticles was studied at high magnification using in situ transmission electron microscopy. The particles prepared by physical vapor deposition onto amorphous-C support films had an average composition of Cu-43 at% Au and diameters of 15-50 nm. In the case analyzed, the larger of two nanoparticles remained stationary throughout the coalescence process while a smaller nanoparticle moved toward the larger particle at a temperature of similar to 573 K. The surface of the small nanoparticle was observed to fluctuate while approaching the larger particle, demonstrating that collective atom process occurs along the particle periphery. The particle also decreased in size during the process, indicating that it was losing mass as well as migrating. Direct evidence of a diffusional flux between particles was observed before the coalescence process. The small nanoparticle coalesced into the large one at a highly accelerated rate compared to its prior migration.