The heat produced by carbon monoxide successively added to vacuum-deposited films of dysprosium was measured in a calorimeter at room temperature. Approximately 363 kJ/mol (87 kcal/mol) were obtained for a wide range of the uptake of carbon monoxide, and the heat was produced at a rather high rate. This indicates a strong interaction of the CO molecule involving its dissociation, followed by the formation of firm bonding of the C and O atoms to dysprosium. The amount of CO molecules undergoing this strong interaction is, however, lower as compared with oxygen. Gradual blocking of the surface by the species arising from CO may account for this finding.