Using an in house developed in situ magnetometer the passivation in diluted oxygen or pure carbon dioxide of a platinum promoted and silica supported cobalt catalyst was studied. No passivation of cobalt was observed after treatment in CO₂, neither at 30 °C nor at 150 °C. The magnetic cobalt metal phase content remained constant during these treatments and decreased in the subsequent exposure to air. The catalyst exposed to 1% O₂ in N₂ was stable in air at 30 °C. However, no long-term stability was observed. The magnetic measurements during passivation in 1% O₂ indicate partial oxidation of the nanoparticles in form of a CoO layer. The degree of reduction decreased from 89% to 51%, which corresponds to a theoretical oxide layer thickness of 1.3 nm surrounding the metallic core with a diameter of 9.8 nm. Upon re-reduction full recovery of the metal phase was obtained. The re-reduction occurred at significantly lower temperatures than the reduction of the freshly prepared catalyst or the re-reduction of a reduced catalyst after exposure to air without any passivation.