Cerium oxide (ceria) is widely used in relation to solid electrolytes in multiple high-temperature devices, allowing metal components in contact to penetrate into the ceria, especially along the grain boundaries. However, few researchers have concentrated on the migration of metal cations at the operating temperatures (e.g., 600-750 degrees C) of these devices. Here, the diffusion and solubility of transition metals are investigated through acceptor-doped ceria grain boundaries as a function of the temperature,pO(2), dopant type, and doping concentration. The use of thin-film samples with high grain boundary density levels and time-of-flight secondary ion mass spectrometry with ppb-level chemical resolution enables an accurate analysis of the concentration profiles of metal species present inside the grain boundaries at such low temperatures. Ni, Fe, and Pt migrate unexpectedly rapidly, and the amounts and types of rare-earth dopants have a considerable effect on the diffusion of the transition metal. Furthermore, transition metals (Mn, Fe, Co, Ni, and, Cu) are present at the grain boundaries at substantial solubility levels of approximate to 10(22)cm(-3), i.e., 1-2 orders of magnitude greater than in the bulk lattice. The observed dynamic behaviors of transition metals present a new perspective on the performance and durability of ceria-containing applications.