The adsorption and polymerization of acetaldehyde (CH₃CHO) have been investigated on clean and CO pre-covered Co(0001) surfaces using the temperature programmed desorption (TPD) method. On the clean Co(0001) surface, CH₃CHO molecules can polymerize to produce paraldehyde with very low efficiency. With pre-dosed CO molecules on Co(0001), the decomposition of CH₃CHO is greatly inhibited. When the coverage of pre-dosed CO is <0.33 ML, no enhancement of CH₃CHO polymerization is observed. However, when the pre-dosed CO coverage is >0.33 ML, the polymerization of CH₃CHO is significantly enhanced during the TPD process. Further results suggest that CO molecules adsorbed at the bridge/hollow sites may initialize the polymerization by nucleophilic attack of CH₃CHO molecules with their O atoms. Moreover, the polymerization product induced by CO molecules is not paraldehyde, but linear polymer chains of CH₃CHO at low CH₃CHO coverages and probably three dimensional polymer structures at high CH₃CHO coverages.