The electrical conductivity of Ca-doped Co_(1-x)O single crystals was measured as a function of oxygen partial pressure, over the temperature range 1273-1673 K. The results were analyzed using Seebeck coefficient measurements, microstructural characterizations, EELS, and X-ray diffraction experiments. From this set of results, we have shown that the influence of calcium on the thermodynamic and transport properties of cobaltous oxide is due to the reducing behavior of these solute cations, leading to both the shift of the Co/CoO phase boundary to higher P(O2) and the formation of singly ionized cobalt cations (Co +) in the stability range of CoO.