  We have examined whether the Co ions crystallographically substitute on the
Ti sites in rutile and anatase Ti_{1-x}$Co$_{x}$O$_{2-delta}$ thin films that
exhibit room-temperature ferromagnetism. Intensities of the x-ray Bragg
reflection from the films were measured around the $K$-absorption-edge of Co.
If the Co ions randomly substitute on the Ti sites, the intensity should
exhibit an anomaly due to the anomalous dispersion of the atomic scattering
factor of Co. However, none of the anatase and rutile samples did exhibit an
anomaly, unambiguously showing that the Co ions in
Ti$_{1-x}$Co$_{x}$O$_{2-delta}$ are not exactly located at the Ti sites of
TiO$_2$. The absence of the anomaly is probably caused by a significant
deformation of the local structure around Co due to the oxygen vacancy. We have
applied the same method to paramagnetic Zn$_{1-x}$Co$_{x}$O thin films and
obtained direct evidence that the Co ions are indeed substituted on the Zn
sites.
