It is shown that dilute niobium doping has a significant effect on the ferromagnetism and microstructure of dilutely cobalt-doped anatase TiO 2 films. Epitaxial films of anatase TiO 2 with 3% Co, without and with 1% niobium doping were grown by pulsed-laser deposition at 875°C at different oxygen pressures. For growth at 10 -5 Torr niobium doping suppresses the ferromagnetism, while it enhances the same in films grown at 10 -4 Torr. High-resolution Z-contrast scanning transmission electron microscopy and electron energy loss spectroscopy show uniform surface segregation of cobalt-rich Ti 1-x-yCo xNb yO 2-δ phase, but without cobalt metal clusters. © 2006 American Institute of Physics.