In situ X-ray diffraction was used to study the reduction of unsupported CoO, and Co3O4 under different conditions of time and temperature, and the effect of lanthanum, ruthenium, or zinc doping of γ-alumina on the reduction of supported cobalt oxide. The reduction of unsupported cobalt oxide (Co3O4) produced CoO, then hexagonal close-packed Co (Co(hcp)) and at temperatures above 400 °C, only face-centered cubic Co (Co(fcc)) was obtained. However, it was possible to obtain a mixture of Co(hcp) and Co(fcc) at 450 °C only when the reduction was performed stepwise. Metallic Co(fcc) was obtained after reduction at 350 °C of supported Co3O4 on doped alumina; however, the reduction of unsupported CoO under the same conditions produced Co(hcp). Doping alumina with ruthenium oxide favors the reduction of Co3O4 to CoO and then to metallic Co(fcc). The crystal size apparently affects the phase of metallic cobalt obtained.