CrO 2 –TiO 2 (TC), MoO 2 –TiO 2 (TMo), and WO 2 –TiO 2 (TW), prepared by heating in vacuo at 1173–1273 K mixtures of MO 2 (M=Cr, Mo, or W) and TiO 2 , were characterized by XRD and EPR. The transition metal ions were incorporated as isolated and clustered Cr III , Mo IV , and W IV and a small fraction as isolated Mo V and W V in substitutional sites. After heating in air or in O 2 up to 1273 K, in TC no isolated Cr V formed, whereas in TMo and TW, M IV was oxidized to M V and M VI , and MO 3 was segregated. The oxidation started at 673 K and gave isolated substitutional Mo V and isolated interstitial and substitutional W V . After heating in H 2 up to 1073 K, TMo and TW reduction process differed: Mo IV gave isolated and clustered Mo III , and Mo IV –Mo IV pairs gave Mo 7+ 2 species; W IV was not reduced to W III ; and temperatures >900 K caused the segregation of metallic molybdenum and tungsten.