Polycrystalline Ti1−xCoxO2 thin films on SiO2 (200 nm)/Si (100) substrates were prepared using liquid-delivery metalorganic chemical vapor deposition, and the microstructure and ferromagnetic properties were investigated as a function of doped Co concentration. Ferromagnetic behaviors of polycrystalline films were observed at room temperature, and the magnetic and structural properties strongly depended on the Co distribution, which varied widely with doped Co concentration. The annealed Ti1−xCoxO2 thin films with x ⩽ 0.05 showed a homogeneous structure without any clusters, and pure ferromagnetic properties of thin films are only attributed to the Ti1−xCoxO2 (TCO) phases. On the other hand, in case of thin films above x = 0.05, Co1−xTix clusters having a soft magnetic (SM) property formed in a homogeneous Ti1−xCoxO2 phase, and the overall ferromagnetic (FM) properties depended on both FMTCO and SMCo–Ti. Co1−xTix clusters with about 150 nm size decreased the value of Hc (coercive field) and increased the saturation magnetic field. © 2002 American Institute of Physics.