Perovskite-type oxide ACo0.8Bi0.2O2.87 (A = La0.8Ba0.2) has been investigated as a catalyst for the oxidation of carbon monoxide. X-ray diffraction results revealed that the catalyst is single-phase and cubic in structure. The results of chemical analysis indicated that in ACo0.8Bi0.2O2.87, bismuth is pentavalent whereas cobalt is trivalent as well as bivalent; in La0.8Ba0.2CoO2.94, cobalt ions exist as Co3+ and Co4+. The substitution of Bi for Co enhanced the catalytic activity of the perovskite-type oxide significantly. Over the Bi-incorporated catalyst, at equal space velocities and with the rise in CO/O2 molar ratio, the temperature for 100% CO conversion shifted to a higher range; at a typical space velocity of 30000 h−1 and a CO/O2 molar ratio of 0.67/1.00, 100% CO conversion was observed at 250℃. Over ACo0.8Bi0.2O2.87, at equal CO/O2 molar ratio, the temperature for 100% CO conversion decreased with a drop in space velocity; the lowest being 190℃ at a space velocity of 5000 h−1. The result of O2-TPD study illustrated that the presence of Bi ions caused the lattice oxygen of La0.8Ba0.2CoO3−δ to desorb at a lower temperature. The results of TPR, 18O/16O isotopic exchange, and CO-pulsing investigations demonstrated that the lattice oxygen of the Bi-doped catalyst is highly mobile.