The behavior of Co-MgO catalysts in the reaction of CO disproportionation was studied in the temperature range up to 800 o C. Two temperature regions that differ by the morphology of graphite produced ( e gg-shells or nanotubes) were detected. The experimental data obtained were analyzed in terms of the phase equilibria a morphous carbon - c arbon solution in metallic Co and g raphite - c arbon solution in metallic Co . Under certain assumptions, the Co o particles are considered to dissolve amorphous carbon under the reaction conditions and transform to oversaturated fluidized carbon-cobalt solution, these fluidized particles being responsible for the formation of carbon nanotubes.