Perovskite-type oxide ACo(0.8)Bi(0.2)O(2.87) (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 ACo(0.8)Bi(0.2)O(2.87) 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/O-2 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/O-2 molar ratio of 0.67/1.00, 100% CO conversion was observed at 250 degreesC. Over ACo(0.8)Bi(0.2)O(2.87), at equal CO/O-2 molar ratio, the temperature for 100% CO conversion decreased with a drop in space velocity; the lowest being 190 degreesC at a space velocity of 5000 h(-1). The result of O-2-TPD study illustrated that the presence of Bi ions caused the lattice oxygen of La0.8Ba0.2CoO3-delta to desorb at a lower temperature. The results of TPR, O-18/O-16 isotopic exchange, and CO-pulsing investigations demonstrated that the lattice oxygen of the Bi-doped catalyst is highly mobile.