Nonstoichiometries of Ca3Co4O9+delta and transport properties of Ca3Co4 +/- xO9+delta were investigated. At 1100 degrees C, Ca3Co4O9+delta transformed to CaO and CoO. The reaction products offer a precise baseline for thermogravimetric analysis. At room temperature, delta in Ca3Co4O9+delta is 0.38, which decreases at T approximate to 450 degrees C, indicating the onset point of the formation of oxygen vacancies, and delta is similar to 0.20 at 900 degrees C. Correspondingly, the average Co valence state is 3.19 at room temperature and 3.10 at 900 degrees C. In contrast to conventional defect chemistry theory in p-type oxide conductors, the formation of oxygen vacancies in Ca3Co4O9+delta has a negligible impact on the carrier density of holes, indicating that oxygen vacancies and the redox couple responsible for hole carriers are in different layers. With control over the ratio of Ca/Co, the phase boundary for the misfit layered structure is between Ca3Co3.95O9+delta and Ca3Co4.05O9+delta. Beyond the phase boundary, the second phase is present, which effectively lowers the electrical conductivity while increasing the Seebeck coefficient.