Using an ultrahigh vacuum (UHV) magnetron sputtering system connected directly to a UHV transmission electron microscope (TEM), we study, in situ, the effect of trace levels of oxygen contamination on the early stages of sintering, coalescence, and morphology. Whereas, nanoparticles produced under clean conditions experience substantial sintering and grain growth upon contact, even at room temperature, particles deliberately exposed to trace amounts of oxygen remain distinct. It therefore appears difficult to form ultraclean nanophase materials, at least for copper, due to rapid sintering. These systematic studies of particle morphology as a function of oxygen exposure shed light on nanoparticle growth mechanisms in the gas phase and thus ways to approach the ideal nanophase material. © 1996 American Institute of Physics.