We report on the electronic and magnetic properties of Co atoms adsorbed on topological insulator (TI) surfaces, such as Bi2Se3, Bi2Te3, and on magnetically doped Bi1.9Mn0.1Te3 and Bi1.98Fe0.02Se3 in comparison to a nontopological insulator surface, Bi2S3. Co atoms are used as probes of impurity affected surface electronic properties of TI and non-TI tetradymite semiconductors. X-ray spectroscopy, x-ray magnetic circular dichroism, and scanning tunneling microscopy and spectroscopy were employed. We found that the electronic properties of Co adatoms do not depend much on the substrate. In particular, no TI specific phenomenon was found. Magnetic dichroism measurements reveals an in-plane easy axis of adsorbed Co atoms on all topological insulators and Bi2S3, but not on the ferromagnetic substrate Bi1.9Mn0.1Te3, for which an out-of-plane easy axis was found.