The precipitate concentration of CO2SnO4 phase in CoO-doped SnO2 ceramics was investigated as a function of oxygen chemical potential during heat treatment. When the SnO2 ceramic doped with 2 mol% CoO was sintered at 1400 degreesC for 2 h in oxygen (O-2) or nitrogen (N-2) atmosphere, a small amount of precipitated Co2SnO4 phase was observed in the SnO2 samples regardless of sintering atmosphere. When the sample sintered in N-2 was afterwards heat-treated at 1300 degreesC for 20 min in O-2, the amount of the CO2SnO4 phase on the surface of the sample was largely increased. On the other hand, when the sample sintered in O-2 was afterwards heat-treated in N-2, the Co2SnO4 phase on the sample surface disappeared. It is suggested that outward or inward diffusion of oxygen vacancies (V-O(..)) in the sample under different oxygen chemical potentials is coupled to outward or inward diffusion of Co-ions due to a defect association of oxygen vacancies with Co " (Sn), which causes modifications of the Co concentration at the surface and controls the precipitation fraction of Co2SnO4 phase at the sample surface.